I disagree BST is the #1 factor determining a pokemon's strength. Kyurem-B has 700 BST, yet it's in ou. Effective BST also plays a role. There are two attack stats, but most pokemon only run one side of the spectrum, so one attack stat has no value in battle. You tell me which is the better pokemon stats: 100/150/100/30/100/100/580 or 100/100/100/100/100/100/600? BST is less important than Stat ditribution. Ubers, Ou, now uu have higher BST because most Legendaries and psueo-legendaries have high evenly distributed stats in all categories. For many pokemon having high attack stat, means lower other stats, but for legendaries, they can have high attack stat and high defenses.While there is no doubt BST is the #1 factor in determining a Pokemon's overall strength (fact: the average BST gets higher as you move up with the tiers), there are other things such as typing, movepool and ability to consider. Perhaps Mega Ray would have been balanced (by ubers standards) if it couldn't hold an item. Or if it didn't learn any good boosting moves. Or if its ability wasn't as good. The thing is, it completely warped the uber metagame around its very existance, even moreso than Mega Gengar (which almost got banned!) and that's why it was banned.
In mega rayquaza's case, it's most of its BST is distributed between attack and special attack. It's defenses and speed are average for ubers. What separates it from everyone else is good ability, 115 speed, and a movepool that includes 120 Flying Stab with no drawbacks and basically every stats raiser move. BST has little to do with how powerful Rayquaza mega is. If you gave Primal Groudon Mega-ray's stats, he won't be half as good.