There's actually quite a bit to unpack, there. While half-orcs receive no consideration from any of the gods (who directly grant 6th- and 7th-level spells) nor their divine servants (who grant 3rd-, 4th-, and 5th-level spells) - being able to receive only 1st- and 2nd-level spells from religious rites and practices (i.e. they can only get to 4th level as clerics) - literally
every race except humans is only slightly better off.
If you look at the class/level restriction table posted previously, the majority of demihuman races were limited to 3rd- or 4th-level spells at most, due to their limited available levels in the cleric class (or druid class, for halflings). So only the divine servants of their gods are giving them spells; their gods simply aren't interested in personally helping them out.
And even then, demihumans are the lucky ones compared to most other races! The DMG makes it clear that most humanoids, while they don't get much further than demihumans in terms of the power of the spells the servants of their gods make available, get far less breadth of spell selection:
Only humans (those who've advanced beyond being cavemen, at least) can rise to the heights that the gods themselves are interested in personally giving them the most powerful clerical magic available. Which I suppose explains why there are so many human pantheons out there, while (at least in the original
Deities & Demigods book) the various demihumans and humanoids only have single creator deities who apparently lost interest in the races they created. It paints an interesting picture of the typical D&D cosmos, and leans into what Gary wrote about how the game was meant to be humanocentric.