If they could get more nutrients, that would put pressure to focus on the nutrients from trapped food compared to sunlight/environmental sources. They currently get enough from the insects to develop traits to get what they do while other plants don't.

I figure it is a structural and/or digestive limitation. Small bugs are comparable weak enough to be slowly trapped or drowned without being able to do enough damage to outpaced the effort on the plant's part. As bugs as plants and bugs scale up their mass in strength are exponential to their length and height so it would be harder to trap them and it would take more energy to create whatever they use to digest. If it wad even slightly better on the return overall, they would scale up until they reached the limit of that benefit.

Gonna go read the article as see if that speculation was right.

Edit: we were both on the right track. They do only need the trace nutrients and evolving the things to trap larger isn't worth the effort, but the main reason is that plants get plenty of nutrients normally and only evolve the ability to trap to cover for deficiencies in the environment in the first place and one plant drops that ability once it grows large enough to get energy an nutrients the regular plant way.