Why don't we recognise Somaliland, but we do recognise Somalia?

There's a number of factors involved namely the lack of lobbying and incentives. Countries generally only recognize a secessionist state if they are opposed to the state from which the secessionists are declaring their independence from. Another reason would be weaken said state which is really not necessary given the fact that Somalia is still in the midst of a civil war. If the US were to recognize Somaliland, it would mean the deterioration of relations with the internationally recognized Transitional Government of Somalia. This is why Somaliland is only recognized as an autonomous region of Somalia for now.