I know that it would be more like an actual mushroom if they would heal more if they were touching "soil" but thats a lot harder to define then "sunlight" is.
Also after a couple of tests I have found some complications I would rather avoid with soil.
- You are pretty much always going to be next to some kind of soil that they player will say is adaquite to grow plant life. Lot's of arguments. Light, not so much so.
- The Jar of dirt... No I don't want my characters dragging around a jar of dirt for extra healing ability. You can't really bottle direct sunlight, can you?
- For fluff reasons as well, the moving from underground to the light, they now worship light that grows the plants that sustain them. I know it's against standard mushrooms but its like an evolutionary thing. Plus it adds the whole good worshiping pelor type, so it has the ability to tie in non nature deities to clerics and paladins, giving more options to the players.