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.

We're used to getting our energy and building blocks from our food. Plants get their energy from the sun, and their building blocks from the air (CO2). They get water and some minerals, but a plant is made up off solidified air. It's like if you could be solar powered living off just air, water, and an occasional multi-vitamin.

Anyway, carnivores don't really get energy from their prey, just the nutrients. It's like self fertilizing.

That's not really how it works. Going from one food chain level to another you lose about 90% of the energy like:

Plants - photosynthesize 100% of solar energy available

Herbivores - eat the plants, losing about 90% of the total energy in the process of breaking it down and making it usable.

Carnivores - eat another animal, losing another 90% aka (1% of total energy)

Plus, consider that photosynthesis is capable of creating all the sugars and that we can convert sugars into fats and proteins and stuff using biological processes (this is essentially why plants need nitrogen and phosphorous to grow, but the generally get that from the soil and really don't need a lot.