So, you buy a new Android phone, try to download the Twitter app, and realise that your 2 month old phone will never be able to run it. Does the phone work?
A lot of custom OS projects wouldn't exist today if consumers believed it did.
Anyway, my main point is that 'twitter' is something most users have only heard from on TV and don't even know what it is. For a regular user, a phone works when you can make phone calls with it, send text messages and set reminders/use it as an (alarm) clock. Then there are fancy features like using it for navigation (which is something they're already familiar with from standalone devices), contact/full agenda sync, and password management; and reading news sites; and post on facebook when they're so bored out of their mind that they really don't know what else to do.
And then there's the uber-advanced, only-nerds-do-this, full phone-centered lifestyle, where as much as possible is automated through phone apps. This is where twitter-client downloading people fall into - web nerds, turtle-neck wearing marketing people and art students. Of course in absolute numbers this is still a sizeable market, but it's only a fraction of the overall 'mobile phone' customer base. It will become more main stream in time, but not now...
How many OS projects are really used? A couple of hundred users, a couple of thousands for popular custom android roms? That's still marginal.