After a tooth extraction why is there a bad taste in the mouth?

Bad taste: Not uncommon but should go away with proper home care like salt water rinsing and general brushing. If it persists then may be an infection so see your dentist.

Dry Socket ?: It is very possible you have a dry socket. An infection, trapped food, and dying tissue can also cause this problem. It's best to have it looked at by the dentist who extracted the tooth.

Concur: I concur with dr davanzis answer. Additionally, there may be some residual bleeding which can leave a bad taste. Also, if other teeth not kept scrupulously clean can leave a bad taste. As dr davanzis said, see your dentist.