To be sure, they never did stop building pyramids. Pyramid building continued throughout the Egyptian Empire. They only stopped it on such a large scale. There are a few quite valid theories why large scale pyramid building came to an end. I will cover the more likely ones.

* To build on the scale of the Giza Plateau is economically unsound. It's simply too expensive. You have to buy or quarry the stone, either way is expensive, then you have to move it to your desired location, which is generally not too close, then you have to pay your laborors, skilled workmen, and overseers, plus give them living quarters. All this drains the economy. It's fine for one or two, but by the time the Giza Plateau was completed, there were at least 7 pyramids in existence.

*As if the enormous drain on the economy wasn't bad enough, it seems as if there was a bit of a drought during the 5th Dynasty. On the causeway of Unas' funeral complex, the reliefs show his subjects as starving, as if in a famine, much as what would have happened if the Nile was not rising as it should. As a mostly egrarian people, a drought in itself can mean the collapse of the economy. Not good if you want to build huge pyramids.

*Finally we come to basic human nature. If you build a pyramid in the middle of a barren desert, it's like a beacon saying "Come steal all my treasures!" Much too tempting for tomb robbers. If you want to keep all your possesions and body in tact, it's better to hide them. Especially when everyone knows your possesions are worth a fortune.

Personally I think it no one reason, but a combination of the three outlined above. During the 4th Dynasty pyramid building was at its height. The 5th Dynasty pyramids that remain are clearly second-rate. The remains are mostly in rubble as inferior building techniques and supplies were used. This could be because they couldn't afford the high quality materials used by their predecessors. This could be due to a combination of run down economy and an unfortunate drought. And as pyramid after pyramid was robbed, often by the workmen who designed it, it was eventually decided that for burial purposes, a more well hidden tomb was better.