Warning: long answer ahead.
Because it only activates when Darmantian is at half health, which will only happen during battle with a very specific requirement.
You might consider Meloetta-P to be in a similar state, but Meloetta's transformation can be controlled by using Relic Song, which will keep her (it?) in Pirouette mode until she switches out or uses Relic Song again.
There is no accurate of controlling Darmantian's transformation. Even if you use Belly Drum, you're likely to get KO'd before doing anything because of the speed drop.
Considering that Darmanitan-Z is a special attacker, it means that if you want to use Darmanitan-Z you need a special moveset. If you use Belly Drum to activate Zen Mode, then you just got a full attack boost for no reason. If you don't use Belly Drum, then do you just intend for Darmanitan to take a hit for Zen Mode to activate? That's not a good strategy, considering how frail Darmanitan is. Then there's the added question, what does it do when it's in normal mode? Do you give it a mixed set? That means EV splitting. How would the EV spread even look? You don't need speed EVs for a Dramanitan-Z, but normal Darmanitan needs them. If you never even get to Zen Mode, doesn't that just mean that its special moves are wasted, and it'd be better to use Sheer Force instead?
Additionally, if Darmanitan-Z's health goes above half, it'll revert back into Darmanitan. The fact that Darmanitan-Z's transformation is so hard to control means that it's difficult to test and difficult to use. Darmanitan also already has a great Sheer Force abuse with Life Orb and moves like Flare Blitz and Rock Slide.
Basically, there's no reason to use Darmanitan-Z over Sheer Force Darmanitan. If you want a special Fire/Psychic Pokemon, go for Victini, or now, Delphox.
As for the Limbo tier... it's not like the BL and BL2 tiers, where the Pokemon there are too strong for UU (BL) or RU (BL2) but aren't quite OU (BL) or UU (BL2) material. Limbo is like a "pending" tier, where the Pokemon isn't fully tested enough to be able to go into a "normal" tier.
What is the Limbo Tier