I think the reason that there are so many in rescues is because boars only (on the whole) come in pairs - whereas with girls you can have as many as space and temperament allows (I now have 15 girls and a boy in one herd and one other girl that lives with her own husboar - then a pair of boars). You need quite a bit more in terms of living accommodation for boars than you do for sows, so there ends up being both more boars that take longer to find new homes.
Until recently I'd only had boars. My first few piggies were boars, then some girls joined (who accidentally got pregnant and had plenty of baby boys) but now I have the space I've gone for a large herd of girls with a neutered male. I think once I get a job when I graduate (or a part time job at uni if I can find one) I'm going to buy a two tier 5x2 hutch and rehome a couple of pairs of boars. I miss having a lot of boars, though my 4 cheeky chappies keep me occupied when the sows are grumpy gits!