I have submitted aircraft photos in the past to airliners.net and had them rejected also. Their conditions and guidelines are not consistent and I've seen some shockers get though (IMO). I've given up and just take aircraft photos for my own enjoyment.
I personally think yours is a beauty and something to be proud of. Go with Dennis' advice - Bombardier may be interested.
I cannot recall the history of your image but I'm guessing it's a Sunstate Dash 8 Q400? I have a couple of 1:400 diecasts of these (
https://store.jetphotos.net/images/P/GJQFA864-300.jpg)
Stephen