Most Helpful Guy
Probably because he had an ulterior motive behind being friends and it's not going to work if you're with someone else. It doesn't really matter what his ulterior motive was only you know he wasn't being sincere and it's good you found out sooner rather than later.
Most Helpful Girl
If you're dating someone new, why do you care if he talks to you or not? He obviously still has feelings for you if he started to ignore you after finding out you're dating someone new. But remember they're always "exs" for a reason.
Move on and enjoy your time with your new guy.