First of all, the WAP specifications require the use of WML. You might have heard of WAP devices that support HTML, but this really isn't the case. There are several other wireless devices similar to WAP devices, but these use either straight HTML (such as devices with the Microsoft Mobile Explorer which supports both HTML and WML. The MME devices are really two completely separate devices in one. Then there's the variants on HTML such as the iMode browsers which use Compact HTML. In short, if we're talking about WAP devices, the markup language is WML.
   

There are many reasons why WML is used in the WAP environment instead of HTML. Currently the most important reason is that WML requires very little bandwidth resources compared to HTML. Obviously, with the introduction of technologies that provide higher bandwidth for wireless devices, this reason becomes less important. However, it will take many years before these higher bandwidth technologies are available globally.

Another important reason is that HTML requires relatively great processing strength to render. Processing strength means power, and on a wireless device that means energy from batteries. Less processor power means longer lasting batteries.

Further, HTML really requires larger displays than the display on a device such as a mobile phone. Sure, it's possible to have large displays on mobile phones or other mobile devices, but the larger the device is, the less mobile it will be. Even with iMode that supports colors and a page layout similar to simple HTML, the displays are already at the maximum size you would want to carry every day.