Among the reasons:
One area the ThunderBolt has always struggled has been with software stability, and that could easily be said about most other Android devices, especially HTC-made phones. Sense UI is bug-ridden and known to be the culprit for lag and battery drains. The likes of custom ROMs proved to be no better as development for the 'Bolt has hit quite a few road blocks over the months.
Little needs to be said here. Android is notorious for poor battery life. Although I tweaked my ThunderBolt seven ways to Sunday (I tried out new ROMs, kernels, radios … everything), the battery life never changed drastically. There was a time or two that I squeezed about 19 hours out of my ThunderBolt battery, but that was mostly standby time. I need something that will last through a day of heavy use, and BlackBerry 6 will no longer cut it for me.