TAI is monotonic clock, and isn't adjusted for solar time of day, it could be considered universal as TAI would be the same between any 2 points, but UTC is adjusted for earths rotation, so a theoretical mars UTC would end up out of sync with earth UTC.

EDIT: info below is incorrect about UTC not being monotonic, as pointed out in thread but is useful for monotonic vs non-monotonic:

In UTC you can jump forward or back, so it's possible to do an operation after another operation but have a timestamp before it, which is bad for many reasons, top being auditing.

do operation one at T0 do operation two at T1 do operation three at T-1

in TAI it would always be do operation one at T0 do operation two at T1 do operation three at T2