For what it's worth I don't think you're doing anything wrong. It's a total crapshoot. When I left active duty I had very similar hours. I ended up flying for a regional for almost a year without a single interview. Almost two years after I quit, I finally got an interview at SWA (but wasn't hired). Of course this was 10 years ago, different time altogether. Not sure the airlines care about your GPA or even what you majored in. Some people get hired with no college or an associates.

My advice is keep networking and go to a WAI or OBAP event. It can't hurt. I agree that it's a frustrating process as you see others with fewer hours seemingly getting multiple job offers. Then again, you have only been at your regional job for half a year - it will happen eventually.