(I already said this elsewhere but...)
Detect when used as a Z-Move raises evasion. While the strategy of Z-Detect might not be inherently broken in itself or used extremely often, we do have to admit that it is gimmicky. Gimmicks are fine in the metagame as well have seen so far, where we have kept smeargle and fear (bad examples but you get the point; gimmicks still exist). The problem is, Z-Detect's gimmick uses evasion. Evasion has been shown as uncompetitive through pre-existing bans, such as the Evasion clause and our own metagame's (what I like to call) "Accuracy Lowing Clause," which bans moves that lower accuracy. Detect not only is one way of violating these preexisting rules put in place, but also brings nothing else to the metagame except a way to increase evasion, as protect does what it does normally, but better (protect has 8 more pp). Every pokemon that learns detect also learns protect.
These things combined make Detect has no rightful place in the metagame based off of precedent, so there is no reason to keep it. Banning it is for consistency, which has been a complaint from many due to a lack of it at times within the metagame.
One might say, "well how come Z-Camouflage and Z-Lucky Chant haven't been banned too?" and I believe there's a few parts to it. One is the difficulty of complex bans, which banning specifically Z-Effects of certain moves is difficult to do. The other is that these two moves have their own effects that do something different than another move, unlike Detect which is Protect but worse (except the Z-Effects are different.)
In the end though, what happened was as Quote said, unacceptable. (Oh my god I'm not sucking off council this time guys!!!) Quickbanning, as has been said, shouldn't be done this far into the metagame, and there should've been a precursor post or something, perhaps "We're talking about banning Detect in council, talk amongst yourselves and post in the thread about it and we'll vote after a week." There's not much wrong that could happen there, except people complaining about the subject of the ban, and why other things aren't being looked at. In the end though, I somewhat take responsibility for this as being one of the root causes, since I brought talk of Detect to attention, so if you want to hate on someone for talking about Detect when other things should be prioritized, it should be me. I just brought it up because I was given hard numbers of the winning chances.
Sorry about the long post, here's a tldr: Banning Detect is for consistency, it brings nothing but a previously banned strategy to the metagame, and the council needs to be more open about this.