Facebook apps, which can ask for any set of permissions from a user at any time, can start by requesting some basic permissions and can request others later when the users trust has been won. In contrast, Twitter apps are very inflexible. You must set what permissions you want and ask for them at the start, with no chance to change them.
If your Twitter app could work equally well with both read-only or write permissions, what do you do? If you ask for read permissions part of your app won’t work, and if you ask for write permissions then some users may be scared to give them to you and just abandon your app.