Back from holidays, million things to do - so some quick answers.
If I missed your question please ask again (after reading the first post and the wiki page).

Most important reason is that I wanted to try something different for the UI. I'm experimenting/learning the Qt components and Designer for some time now and wanted to try some new (for me) things.

Second, some times in development it is better to start from scratch. fAircrack uses too many hacks (e.g. to gain privileges or pass variable values between methods), that it is faster to rewrite it. (At least that was the case when I saw it last time, which was many versions ago.)

Third, I asked the developer when he was in the first versions if he wanted to join forces but he politely declined.