Pretty sure they modeled Paladins off of the concept of Knights and Codes (of Chivalry) rather than as a Knight Templar (which was 3.5e and before). I personally like that Paladins are bound by codes now, it distinguishes them over just being a half Cleric with a sword. That said, even in the lore, some Paladins are known to be religious, so I don't understand why a deity isn't selectable.
Actually Paladins in pretty much all media are not just Knightly Orders but religious nightly orders dedicated to gods and until 5e were tired to gods. While they separated it mechanically it is rare to see people play paladins without them
After all, there are dozens of gods in the world of the Forgotten Realms, and it is not realistic to create a special paladin for each of them.
Its actually quite common to create paladins for specific deities. They are often defenders of the faith though traditionally there are some deities that are more likely to have large numbers of Paladins like Tyr, but that doesn't exclude others if they feel the call of that deity and the path of being a paladin
