I guess
A plant loses a feature means this feature is a kind of disadvantage in its environment.
Meanwhile, what it gets from the lost feature can be got from its environment.
I think in the leafless orchids' environment, leaf is a disadvantage,
maybe it's a dark place, and hard for plants to do their photosynthesis, and the orchids can get carbon and water from the environment, for example, from soil, instead of leaves, from which orchids usually got carbon and water.
Maybe it's not a dark place, but whatever it is, leaf must be a disadvantage for these orchids.
This is just my hypothesis, but it's sure that losing a feature must be reasonable, and the orchids can adapt themselves to the environment better, through losing a feature.
