I see where you going with this, but I think you getting your concepts mixed up. Don't think it had anything to do with time signatures.
In most rap beats the snare falls on the 2 and the 4 and there's most always a kick on the down beat (the one). Those are the most emphasized beats in the instrumental, and a lot of rappers will let their rhyme falls in a familiar and consistent place based on that back beat.
A rapper like 2 Pac usually rapped in 1/16th to get that jumpy hi hat feel, but he still let his rhyming word fall in a familiar place, that's probably why it's easier to lay his acapella for you.
Rakim and big were more experimental with their flow, in the tradition of jazz horn players who improvised and the culture of that world celebrated experimentation. The whole idea was to defy the expectation of the listener thru emphasizing off beats (not of the beat, but off beats as in not the 1, 2 or 4), use of phrasing, etc. So I see why you'd find it harder to place their acapella over an instrumental, because their rhyme patterns often defy expectation purposely. In other words the rhyme might land on the snare for 1 bar and then suddenly move to a different beat. The truth is, if you're very good at laying acapella, you can actually lay a big or rakim verse in more than one place and it still sound on, it'll just be emphasizing a different set of beats. I've heard remixes do this and it gives a verse a whole different feel.
These guys different.