The ones that do are mostly doing it because it's a legal way to keep the riff-raff from moving in and ruining your building's NPR-listening vibe with a bunch of twangy country or loud-ass hip-hop.
That's a bit uncharitable. Landlords do credit checks because if a tenant cannot (or does not) pay his rent, the landlord stands to lose thousands of dollars. It can take months to get a non-paying tenant evicted, during which time the landlord still has to make all mortgage payments, entirely out of his own pocket. Furthermore, serving a tenant with an eviction notice is no fun for either party, and a pissed-off tenant may well cause thousands of dollars of damage to the landlord's property before he leaves -- again, money that the landlord will have to pay out of his own pocket before he can put the unit back on the market.
So yes, there are really good reasons why a landlord would want to vet a potential tenant thoroughly before giving them the keys to the property. The landlord is taking a big risk every time he/she rents out a unit.