There're a lot of inconsistencies about the tribal tags, I'm not even going to try to list them.
But the reason /IMO/ is not really an omission but intentional game designing. Usually the tribe makes the minion more valuable which is not always working with the desired mana cost. Sometimes it allows interactions which devs doesn't want to be available, or consider strong/oppressing.
Keep in mind they plan the expansions at least an year ahead so they take in consideration things we can't even imagine. And once the card rotates out of standard the probability of a change /e.g. adding tribal tag/ drops dramatically.
Just took a look at the art of the Bogstrok Clacker and wanted to point out /IDK if you're a WOW player/ that the creature is a makrura which is considered as humanoid in the game. The Golakka Crawler is just a type of crab in Un'goro and is a beast. Anyway, you're right there're inconsistencies, especially with the elemental tribe..
Pretty sure Loatheb is a fusion between a bog beast and pure plague, so the right tribal for him would be undead if that ever makes it in I think. Edit: And Ixlid WAS an elemental but got the tag removed to pull him out from random pools I believe.
I remember when they made those statements for arfus they basically said "Yeah this shouldn't be a beast either but we like having it in the pool for Deathstalker Rexxar's hero power, so it's staying."