Well the thing works like this now: Demonic will spot a new mob, to have an accurate reading on TTD it waits a bit to see how fast that mob's HP will go down I think it waits till mob reaches 99% HP, then it starts calculating TTD. We had a problem with Doom especially when we were already in meta form that as soon as a mob would spawn it would cast doom on it whatever it's health would be since TTD on spawn is 999 seconds and it concludes that Doom is worth casting. That is why we made Doom have minimum HP setting so we can ignore mobs that will die fast. While it is good for Doom there is a problem with Cata if we would make decision how to cast it based on HP setting since that is AoE and in raid mobs can die in 10 seconds or 20 seconds and all depends on your raid composition, not on HP, at least not in BRF since on Darmac mobs live around 15 seconds while that may sound good to cast cata in meta form it really isn't, since our cast time is 2.5 seconds on cata and by that time mobs will be on 70% so if cast in meta we wouldn't benefit from it. Same thing is on operator Thogar, those low HP mobs live very short, 10-12 seconds, while others live longer. On Thogar it's all about your raid composition and if you would set HP to for example 2M then how would demonic decide when you have 4 mobs fitting category of normal cata casting and 3-4 mobs fitting category of meta casting?
In all of BRF I haven't seen a single fight where cata is worth casting in meta form unless you are Cata specced for Iron Maidens, and if you do that you're not helping your raid.
Automatic decision of Cata casting in meta/normal is simply too much problem for the routine and it requires other changes to be done to the rotation that would possibly cause more problems. While we always try to make routine perform best for everyone sometimes it's not possible to make it to perform perfectly in LFR and in Mythic same time. That is why we have settings that help us tune it for particular boss or raid setup.Also when you get 4pc Tier17 I think its much more beneficial to cast Cata in normal form on heavy AoE since of high chance of corruption to proc free HoG/CW which you can use on that same pack for more dps.