Presumably the reason they changed it is becase "twttr" sounds stupid. :P
Every time I see a name like this, I think: Great, now what do you do when you tell someone to go to your domain? How do you pronounce Twttr or fllwrs in conversation? Do you say "Twitter without the i or the e. Followers without the o's or the e."? That's dumb. You shouldn't need to explain how to type your URL. Chances are I will forget. To complicated.
It's a stupid naming pattern. People need to stop it. -_-
Flickr I think was the original inspiration. Basically no one wanted to pay for the real domains like they did in the first bubble and it was basically a red flag to spend your money on one.
