In 2002, Bill Gates predicted the majority of PCs shipped by now would be tablet PCs, but their numbers are still just a fraction of the market and account for less than 2 percent of all laptop computers shipped.

While tablet PCs are accepted in increasing numbers by medical personnel, insurance adjusters and users in other vertical markets, why haven't they gone mainstream?

Analysts cite at least eight reasons why tablets haven't done better, including their higher cost compared with standard laptops (up to £200 more), problems with touch technology and handwriting recognition software, and a shortage of suitable applications.

Recognising those problems, tablet PC makers recently introduced new second- and third-generation devices in both the convertible and slate form factors. More new models are expected later this year running the Vista operating system, which will eliminate the separate Tablet PC Edition in Windows XP and the resulting inefficiencies, analysts said.

"Tablet PCs remain a niche product in the marketplace, used predominantly in vertical applications," said Gartner analyst Leslie Fiering in a recent conference presentation. The tablet PC's "appeal to horizontal, mainstream users will continue to be minimal" because of higher costs, an "immature" touch interface and other factors, she added.

Fiering, who started following tablet PC trends at their introduction in 2002, made the strongest condemnations of tablet PCs of five analysts interviewed for this story, but she also offers the greatest optimism.

In an interview, Fiering said she is still "bullish" about what the tablet PC can become. "It's important technology which is slow in coming," she said. "A lot of things have to fall in place, but it's real."

The tablet PC market has tracked fairly closely to Gartner's forecasts, although well behind what Gates was hoping for. Microsoft predicted sales of 1 million tablets in all of 2003, she said, while Gartner predicted sales of 230,000, about 5,000 higher than actual shipments, she said. That 1 million mark was finally reached in 2006.

Several analysts said they have had to consistently revise their forecasts downward for tablet PCs. About 18 months ago, market research firm IDC forecast 7 million tablets would ship in 2010, and last year revised the number down to 5 million, said IDC analyst Richard Shim. Gartner expects the 5 million mark to be hit in 2009.

IDC puts total tablet PCs at only 2 percent of all laptops that shipped last year, a number that might exceed 3 percent this year, Shim said. Other analysts said the amount is less than 2 percent.

"The forecasts have definitely been reduced since they first came out," Shim said. "A lot of hype has been built ... by Microsoft."

Slowness in sales of tablet PCs means nothing to many happy users, however. For example, at the Altoona, Wisconsin police department, two Lenovo ThinkPad X41 Tablet PCs have been in use inside police vehicles since 2005, and the department is planning to add as many as 15 newer models in the next year, said officer Dana Brown, manager of the department's technology initiatives.

"There was a $200 premium over a standard laptop, but we wanted the versatility, and it has definitely paid off," Brown said. "We're getting away from the keyboard ... Tablets are more than a niche for law enforcement."

Proponents aside, here are two reasons why tablet PCs haven't moved into the mainstream - tomorrow, six more!

1. The price is too high

Compared with a typical laptop, a tablet PC can run from $200 to $300 more, meaning most tablet PCs run from $1,200 to $1,800 in the US, depending on functionality, Fiering said.

Gartner predicts that the price differential will not come down in 2007 and might not come down in 2008, depending on the prices of screen digitisers, the technology behind the screen that turns a touch from a finger or a stylus or the electronic impulse of a special pen into data that can be stored.

Fiering said that digitiser prices have proved "remarkably stubborn" and have not dropped as components tend to do in the PC industry over time. Part of the problem is that suppliers have not geared up for massive production, held back by fears that the market might not do as well as predictions, Shim said.

Even with higher prices, tablet PCs have sold a little better in vertical markets than earlier predicted, said Roger Kay, an analyst at Endpoint Technologies in Framingham, Massachusetts. In settings such as health care or manufacturing, tablet PC users have found using a pen or a finger to check boxes on a standardised form is easier, eliminating the need for the keyboard, he said.

The leading applications that benefit from a tablet PC are for clipboard replacement and handwritten annotation, Fiering said. Filling out forms for inventories, surveys or patient care has proved the most popular.

Also, insurance adjusters, contractors and students can use tablets to take notes. For example, non-text input for college chemistry or math majors who use symbols and formulas has been popular, Fiering said. Note-taking is also important in work situations. Salespeople and doctors, for example, must interact with another person, keeping eye contact while writing instead of typing on a keyboard.

Still, the price premium for tablet PCs is a real problem for further adoption and not helped by the fact that conventional laptop prices have come down in recent years, Shim said. When first introduced, tablet PCs cost as much as $800 more than a laptop, compared with $200 to $300 today, which shows there has been some improvement, he noted.

2. Touch technology hasn't caught on

Despite the advantages of handwriting and touch to some vertical market users, touch technologies with a finger or a special electronic pen used in tablet PCs have not caught on in the mainstream.

The main reason they haven't caught on is that the keyboard is already widely used and is growing more acceptable to use every day, said Ken Dulaney, another Gartner analyst. "Many people have learned to type out of school," he said. Text messaging and instant messaging only contribute to the willingness to use keys for input, several analysts said.

Also, touch technologies in early tablet PCs have not been that accurate, unless the application is forms-based with boxes to check, Shim said. Touch also has not been promoted by vendors.

"Touch is intuitive, but there's so much more that we could do with it, and not enough applications have been written to take advantage of touch," he added. "Typing is a well-accepted input system, and keyboards are here to stay. We seem to become more and more attached to them."

Tomorrow: Six more reasons