Maybe because your traffic quality is low( non-english speaking). So they dont shows ads to your sites anymore.
adbrite ads are not contextual. So the ads are not always very relevant to your site content. But still they try to match it as best as they can.
Adbrite is different, its not like adsense, adbrite ads are not contextual ads, its for people who want to advertise and run ads on your site, that why you see all the time *Advertise on this site, powerd by adbrite*, when you have decent amount of trafic they show network ads.
adbrite different with adsense, adbrite is non contextual it means an advertiser can put ads on any website he wants, no matter the content