But not that elves and humans can't multiclass cleric but half-elves are the masters of clerical multiclassing.Early D&D has some weird stuff going on with elves and gods. For one thing, elves can't be raised, only reincarnated or resurrected. I believe this is because elves don't have souls, but instead have spirits (whatever the difference is supposed to be). So in that view, it makes sense that it'd be harder for elves to have connections to their gods, and that doing so would require more dedication than the dabbling of a multi-class character.
If elvish blood/nature makes it harder to connect to gods as a cleric then the half-elf multi class situation is particularly weird. Also that 1e NPC elven clerics can be higher level than half-elven PC or NPC ones.
In the 1e PH it is mostly a PC human thing with half humans (elf and orc, both spirit things) being able to as well. Other demihumans cannot be PC clerics. This reflects Oe where clerics could only be men, MUs could be men or elves, and all PCs could be fighting men.
Once you have the 1e UA/2e PH PC demihuman options the elven restriction on cleric multiclassing stands out as weird.