Many players and fan designers have been forever frustrated by the fact that in MtG, "Tribal" is a card type instead of a supertype. Having been digging at this for a while, I have to a conclusion: there is literally and absolutely no rule justification for this.
The only argument that was ever offered is "subtypes must be associated with a card type, not a supertype". He re's a typical formulation from today. Tabak has never explained it any further despite many pressing attempts t getting a better answer.
However, this only begs the question, because this itself has no rule-based justification whatsoever other than (if you squint really hard) the fact that Tarmogoyf and a few, select cards would be functionally altered. However since a) they forgot about THAT when creating tribal in the first place and b) this would be a minute amount of disruption to older cards compared to what happened by eliminating damage on the stack or mana burn. (And you'd think they'd jump at the chance to rein in Tarmo a little without actually altering cards or the banned and restricted list.)
So, I ask of you: come up with a rule problem (and as I made it clear above, a card whose power subtly changes is not one!) or contradiction that arises out of making Tribal a supertype. Please. Please tell me Wizards have a reasoning instead of just stubbornly pretending they didn't make a patently idiotic design mistake and then tried to cover it with what amount to "because we say so".
An example of a rule issue would be if it could somehow cause a creature to have undefined power or toughness, or cause an instant/sorcery to be face-up on the battlefield, both situation which templating and rules carefully thread around to prevent their possibility.
Yes the question is ranty because Tabak has been frustratingly stubborn on this issue, but it's still a legitimate rules question.