"not many people use it." This is wrong, as already Fizz told, it has the highest usage of almost one in every two teams, and viability is well one of the best and most viable Pokemon in the tier, ranked at S in the viability rankings alongside Necrozma Dusk Mane and Zacian-Crowned. Here's a explanation from Smogon:
Mega Rayquaza sports immense raw power, a stellar offensive movepool, and the ability to hold an item, making it one of the most lethal wallbreakers Anything Goes has to offer. Moreover, its access to potent boosting options in Dragon Dance and Swords Dance further strengthens its wallbreaking prowess. Mega Rayquaza also boasts a solid Speed tier, notably outspeeding huge threats in Yveltal, Xerneas, and Primal Kyogre, which helps it in finding setup opportunities by forcing switches. Dragon Dance turns Mega Rayquaza into a potent wallbreaker as well as a late-game cleaner. Swords Dance may be opted for instead in order to turn Mega Rayquaza into a destructive wallbreaker, allowing it to OHKO the bulkiest of threats such as defensive Zygarde and every support Arceus form.
As you already see, giving Mega Rayquaza a free turn to Dragon Dance, not having Ditto, Zygarde-Complete, Marshadow (ensuring it has Sash with no Stealth Rocks), Lugia, etc. means you are totally wrecked up by Dragon Ascent and V-Create/Earthquake. Alternatively, it can be used along with Focus Sash as a good lead, and a huge boost it received was Scale Shot. While it's not a good idea, it makes opposing Ditto locked up and easily revenged killed by your ally Fairy types, while defeating Smeargle, Viviliion, etc. and boosting its speed while giving some damage. It's also your only way to run a good Dragon-Type move if not running mixed with Draco Meteor. Even Choice Scarf can be totally unpredictable, getting it in your enemy's crowned swords face makes it very viable.
It was the king of AG in Gen 7, even though now Zacian-Crowned has higher viability rankings than it (remember that Pokemon in higher ranks of +A, are ranked from top to bottom according to their viability), it's the second most viable Pokemon and the most used Pokemon. It has so good movepool combined with Life Orb, Dragon Ascent OHKOes many important Pokemon. It checks: Necrozma-DM (Ultra one beats it though its less common than Trick Room ones and defensive), Yveltal (non-dynamaxed, Life Orb), Xerneas (gets KOed by Dragon Ascent with Life Orb if not HP or defense invested), etc.
Hope this helps!