divine word seems from its spell description to derive its powers from the song of creation, that thing all bard spells supposedly come from and that all bards try to snatch up and learn, and while the ally from planar ally can indeed be your particular deity the other possible allies you can beseech for aid seem to be more warlock patrons than divine forces. So then why were these spells put on the cleric spell list and not the spell list of these other classes, and why did neither class get their spell available via tasha's cauldron of everything? Also why isint thaumaturgy available to warlocks, yes they are arcanists and sages but they are still influenced by supernatural power right? seems only fair.

and on a similar note, why was prismatic wall considered so necessary that it needed to be on the bard list by default when Divine Word was not? And why does being directly connected to a god allow me to channel divine energy into bargaining with non-divine entities? 

I can't say anything about planar ally or thaumaturgy but I guess divine word is to do with the fact that Bards are considered arcane casters and clerics are considered divine casters?? I guess you could also say that Divine word is derived less from the song of creation specifically (especially since it doesn't specify the song of creation) but instead is derived from divine energy from when the gods first came into existence.

I fully think that the other two should be available to warlocks