I guess this depends on if they lost functional roots or not. If they are like bromeliads that lost water uptake in their roots (which instead take up water and nutrients through trichomes in their tank) then they probably don't care about how much nutrients are in the soil. I'd think that as most bromeliads are epiphytes without any real type of soil that that is the reason they lost this functionality. And that many species of carnivorous plants are usually just growing in nutrient-poor soils. So eating animals would just be a way to get more/sufficient nutrients but that it might be still useful for them to have functional roots.

But this is only speculation. Maybe someone else has more tangible info?