Errr - Price (and amount of BTC awarded per block) affects how many people mine, not the other way around. If the price increases, more people will mine because it's more profitable. Also worth noting - more people mining doesn't increase the rate at which BTC is "produced," just who owns newly-minted coins (rather, 50BTC blocks are split among more people if more people mine [assuming all have equal hardware], but BTC won't be awarded faster or in larger quantities just because the network has more hashpower behind it).

It's also Winter in US/Canada, so people might mine until mid-Spring because the heat GPUs produce is also valuable (when it's hot, the heat's undesirable and not only wasted, but might be costly for people who try blowing the hot air outside). I don't think there's data on hashrate distribution by country, but I'd assume US/CAN make up >25% total. There are some people who speculate BTC price will increase, so mining now at a loss might result in net gain mid-term or long-term - an idea ciphermonk notes. GPU mining also isn't necessarily unprofitable -- some people mention having electricity included with their rent, but even for those who pay for electricity by KWh, the price is high enough and difficulty low enough where some can still make a small profit. Some people actually mine on principal, too, thinking increasing network security (however minimally) is itself valuable. Finally, FPGAs are in the wild, and while they probably don't make up too large a portion of total hashrate, it's worth noting those won't be unprofitable to operate until the ASIC backlog is cleared by manufacturers.

Probably a lot of other reasons I didn't think of, and there are plenty in this thread already, but anyway - there are many reasons people still mine.