I can't answer but i can give you my train of thought.
In my opinion the thing is that all decent fighters know almost all techniques.By that i mean that if you were to give them a test and ask them how to do or counteract or even perform something,most would know how to answer or do it.
But the difference between learning boxing combos or how to perform a spining kick and being an elite striker lies on knowing how to set everything up,how to read your opponents striking on the spot etc.The people with a striking background have been doing it forever and that time simply cannot be matched by someone who started later.A muay thai guy has been conditioning his shins since he was a kid for example.
On the other hand take a fighter with a wrestling background.That person's muscles have been growing according to his training since he was a kid.To put it simply their asses are different.
Also I cannot back this idea up fully,but i think that boxing is more instinctive to humans.Put two untrained people in a fight they will start swinging.So in a sense the most basic striking techniques come easier to wrestler that basic wrestling grappling to a striker.Plus kicks in general are not the most prudent choice most of the time.(with the exeption of leg and body kicks)
One great example of a weird combination are the diaz bros.They have boxing and bjj(and neanderthal chins

) which i think they give almost equal importance too.They lack the middle part which is tdo/tdd.So they will box their opponent up,till they are not fighting at 100% and then take it to the ground and finish them.(Nick has been a bit more passionate about his boxing though later in his career...)
Also i think the biggest advantage wrestlers have is point fighting.An elite striker will be required to be much more dominant and mess his opponent up,all the wile taking risks,in order to get a decision.But a wrestler can win a match 50-45 and leave both guys able to fight the next day after a good sleep.