why cant i buy this domain

Expires on: 2010-05-04

well that date as passed but its still saying taken

it is most likely registered or renewed and the place where you are checking it has not updated yet!

Most domains have a 30-40 day grace period now while they contact the owner and ask about renewal. This could be the case with the domain you are trying to purchase. It could also already be backordered if it is really popular in which case you will end up in a bidding war after contacting the registrar or host.

There are some factors why you couldn't buy the domain
It could be already backordered, or taken by others
You can whois domain to make sure

Its exactly what cadkins45 said, all domains have a holding period on them which is 70 days, there is a 30 day grace period where the owner can by them back at the original rate, after this the rate increases.. Eventually the domain will get released into the public domain again, but there could also be domain snapup companies that are waiting to by it and you would then need to enter into a bidding war with seal bids which can get costly, or you might get lucky and just be able to purchase it straight off if there is no other interest.

Best of luck

maybe it's not updated?

thanks, hopefully they wont renew it