Even when playing theater of the mind (which I've done for years in previous editions), I find I want the occasional sketch of a complex environment, handouts of NPC portraits or mood-setting scenes (or of complicated potential trap areas like what was provided in the modules of yore). Then you might want dice rolling - to see the results of an important roll, just to feel like you're at the table. Or you might need to access various character sheets (including monster stats) all at once in various windows. To my knowledge, DNDB doesn't do any of this. It can supplement other things by providing basically a glorified form-fillable PDF character sheet, but not much more. I don't understand why wanting a one-stop solution to all of this is a bad thing. I would expect even most of you DNDB supporters would recognize needing these features at your table.
Concerning making characters, here are a few issues. Limited number of slots. Having to add a character to a campaign before you can test builds with purchased content. Pressing the back arrow takes you out of the character builder entirely. Having to sign in with two accounts (DNDB and Twitch). Additionally, my characters have come out wrong from time-to-time.
But mostly it's that I don't want to pay for a third time to get something that Roll20 already does, when DNDB doesn't offer the functions I listed above.