I don't see how much smarter you need to be to open someone up with Mileena. Lao Hat + F4 has been one of if not thee most single effective mix in Mortal Kombat 1 competitively. It's beaten every top player at some point, including as recent as hourglass beating both NinjaKilla and Unjust back to back over the weekend. I ain't seeing Nitara do anything really competitively, which is why I say that.
Yes, but the rewards for lao hat into F4 are also minimal, where Nitara's S3 into Sonya overhead S34 into square wave are a half health bar depleted - both options fully safe and unfuzzyable, mind you.
Mileena needs to space herself much more precisely to whiff punish and must have reactions on point, while Nitara just needs to air dash back into divekick and is instantly in your face mixing, from anywhere on screen.
Flawless blocking Mileena's 12 will neuter half her gameplan, and while Upblocking Nitara might do the same, she can bait those much more consistently.
Tl;dr: Nitara can start her gameplan much faster and safer than Mileena, which needs to have at least
some degree of methodic positioning before enforcing her tools (even though yolo ballroll is a thing).
There about as many Nitaras in top8 in R1P's Arena as there are Mileenas there, from what I can recall, too - which is to say: few ones. It might be the case of there simply being less interest in playing her at higher levels, though that doesn't necessarily mean she is worse than Mileena. Sindel/Tremor is certainly
not, and we also see few ones beating the famous top players.
At the end of the day, I can offer only insight on what I know considering these are my 2 most played characters, but tier lists are of course subjective at the end of the day.