As a politician trying to reach members of the public, including those who could possibly be swayed by you, the last thing you want to do is entirely remove yourself from a platform with a very large reach.
Wanting people to leave Twitter is all well and good, but you can't discount the fact that so many people are still on there, and refuse to spend the time setting up new profiles on entirely different sites with smaller userbases. For politicians, reach is key, and Twitter still has the users.