The specific nutrients it gets from the bugs are nitrogen and phosphorus, which plants normally absorb through their roots. It evolved carnivory to compensate for the poor soil in its native habitat instead of developing its roots like other plants in the area, so it can't pick up the nutritional slack with its atrophied roots.

In addition, every time a leaf closes and tries to digest what it caught, it uses a lot of energy. Flowering always is a big strain on a small plant, no matter what species, so when this strain is introduced the number of carnivorous leaves becomes a difficult risk/reward calculation, and plants are not known for cleverness.