It actually does take additional development to make it work on android or ios. And if you use either of those platforms you have to use the specific browser that is supported on that platform. You cannot do everything in the mobile version that you can do in the desktop version(just like you cannot use all the features of a lot of desktop optimized websites in a mobile browser). The limitations aren't even just because of the browser, some are due to the way the underlying operating system works. I forget what the limits are(I know one was voice chat didn't work), but they have to do with the different ways that the browsers handle things. And none of the supported mobile browsers are staying the same. They are all being upgraded constantly, and some of those changes break the way roll20 works. The extra effort that goes into keeping the very few supported mobile browsers working is why the feature is subscription based. If it was as simple as just a few static UI changes, I think it would very likely be a free feature, but the way the software on mobile devices is right now....nope. Not gonna happen.