This is an interesting topic and unfortunately I don't think P&P gives us enough information to satisfy our curiosity...

I am intrigued by Lady Catherine's statement which makes it clear that an entail was a strategy that a land-owning family could use if they thought it necessary:

"I see no occasion for entailing estates from the female line. It was not thought necessary in Sir Lewis de Bourgh's family."

We don't know when Longbourn was entailed (was it Mr. Bennett's father's decision, or an earlier generation?) or why. Maybe to prevent its passing to a daughter and her present/future husband, maybe to prevent its being divided up and sold off in parcels? Austen doesn't give us information to understand the entail much better than Mrs. Bennett does ;-)