Which makes it redundant with high protection/armor boots.
(Unless you're no damaging, I suppose. That's a pretty narrow niche, though.)
It is infinitely easier to make it a limited buff that, in passing, makes it good at crossing lava/acid with enemies around, than it is to try to force enough crossing of acid/lava(through RNG map generation of required crossings) so that Envirosuit carrying would be a good thing, without randomly creating unwinnable runs where you draw four large acid rivers early and die.
It's very low odds right now to get maps that require heavy damage from terrain crossing. That's a good thing; It's way too hard to balance without creating frustrating deaths("How was I supposed to survive that? I haven't even seen an Envirosuit yet/used them all up!"). But it does mean that people don't carry Envirosuits a lot.
And even if they did, why would you carry them over a pair of, say, Fireproof Plasteel, which will work for far more crossings? (Honestly, Plasteel's usually enough.)
Personally, I didn't think that was an issue; I've played Dungeon Crawl, and I'm used to all sorts of items having ridiculously narrow niches in order to water down the item generation system a bit.
But if people do, doing an overhaul of boots and terrain generation and item generation to make Envirosuits more useful seems harder than making them somewhat better at doing things besides terrain crossing.