For example, a 4 line display is very different from an 8 or 12 line display. A 2K deck size is very different from an 8K deck size. Identifying and developing such compelling uses for the small screens was always going to be difficult. In the early days of WAP, the limited screen size was quoted as the most likely reason for future failure. True enough, small screens limit the usefulness and make designing compelling solutions more difficult.
Limited solutions means a limited market.
As screen sizes get bigger more options become possible. However, with a bigger screen, many of the current problems don't go away. In fact, they get worse. We will still have the problem of many varied form factors and consequently many different renderings of the WML output. The content in that output will get more and more elaborate as the temptation to place more and more on the screen becomes overwhelming for the marketing people.