Mr. Bennet may have been away at school, the future Mrs. Bennet may have suddenly matured into a very pretty young lady from being a gangly kid ;-), or various other reasons. Also, the point of entails was not only to keep property in the same family, but to keep it under the same family *name* (in which case of course it failed with Mr. Collins!) I assume that Mr. Bennet *did* inherit Longbourn from his father (by far the likeliest scenario), but somehow, I suspect his father had already died by the time he fell in love with Miss Gardiner. IMO, if his father (and possibly his mother) had been around, they might have had something to say about falling in love with a pretty face! Also, and this is just my speculation, I can see him being more vulnerable to falling in love if he had only recently lost his parents and felt rather alone in the world, or on the other hand, if he and his father hadn't gotten along and he was rejoicing in his newfound freedom to choose for himself!