This website explains it much better than I can, but the tl;dr explanation is there's no "privileged frame of reference" in relativity. Even if two frames of reference can be arranged to agree on a causal sequence involving FTL phenomena, a third observer can be constructed that perceives the sequence of events happening out of order; they get the light from effect before they get the light from cause.
This creates nasty phenomena that we don't seem to observe in nature (i.e. if the third party observes effect before cause, they can interfere with cause. Everyone loves a good temporal paradox ;) ).
This creates nasty phenomena that we don't seem to observe in nature (i.e. if the third party observes effect before cause, they can interfere with cause. Everyone loves a good temporal paradox ;) ).
http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-ti...