why do older blackberry phones sell so cheap?

I am thinking of buying one but i cant understand why they sell so cheap. is there something that i need to know before i buy one?

because they're old and they probably aren't compatible with most of the new programs

Oldness, everyone wants the new models, so they have to sell them cheaper so people wil get them because if they sold them at the same price of the new one, everyone would go for the new one...

its probably because there are newer models on the market, and they have more capabilities then the older blackberrys.