Many apps don't work when moved to the SD card. Developers who know that their apps won't work from the card just turn that option off, so they don't get comments and emails about the app not working. (And, as more and more people use app-building apps, because they don't actually know how to develop apps, and as more and more of those app-building apps are written so that the apps they build can't be run from the card [because the people who "develop" the app-building apps don't really know how to write apps], the more apps you'll see that can't be moved to the SD card.)
The current trend is to make phones without external cards at all, but until then we'll be able to move fewer and fewer apps to them. (People don't realize that by doing things we're not supposed to do, we only shoot ourselves in the foot. If SD cards had always been used only for what Android intended them to be used for - pictures, music, videos, etc. - there never would have been a problem. We (those of us who didn't understand this) caused it - now we're complaining that we're not being allowed to do what we shouldn't have been doing in the first place. Google used to warn us to not expect all apps to work if they were on the SD card, and to never move widgets to the SD card but they were ignored, so they stopped warning us, they just stopped including SD cards in phones. (I wouldn't be surprised to see Android come out with no support for SD cards, whether the particular phone manufacturer includes an SD slot or not.)