Because they want to deliver a good experience to every user and redirecting to the simple version is the easiest way to do that. If they served mobile.twitter.com to Firefox for Android users they would have to test the site in Firefox for Android and since only like 12 people use Firefox for Android it's not worth their time to do so.
They're going to have to deliver (and test) one version or another. Or if they're not testing, they might as well send an untested normal version instead of the untested simple version.
