An in-universe explanation for why they could not be combined is available, at least retroactively but based on TOS itself, including things discussed in dialog: The tricorder evidently could not retain all its functions and be miniaturized to be smaller than what we saw*, an item that was too big to attach to a utility belt and needed a shoulder strap to facilitate its portability, whereas on the other hand the communicator was intended to attach to a utility belt, be carried ubiquitously by all members of a landing party**, and be inconspicuous in certain scenarios.So, increasingly, are smartphones.
It's pretty dumb, after all this time, that the communicator and the tricorder are separate items.
In TNG, the communicator got so miniaturized that it was brooch-sized and worn as one. Everyone always had one on, so there was no need to include communicator circuitry in the tricorder, though you'd think that the cost of including it anyway would be about zero for them. On the other hand, I'm not sure that we could establish canonically that tricorders were not also communicators.
* - or it would have been, yes?
** - females in minidresses sans utility belts notwithstanding