They haven't yet. Kyuranger is essentially a space team of 12 Rangers:
I agree they win, but in fairness, the MMPR could beat him in a straight up fair battle, too. It's just that Drakkon's schtick (both in the future where he conquers Earth, and the current storyline), a lot of his plans are dependent on copious amounts of prep-time to allow him to consolidate enough power to ambush and overwhelm our heroes. Indeed, in his original timeline, Drakkon basically helped Rita build her power to heights unseen in the main timeline, and he jumped on the opportunity when Zordon tried to create the White Ranger powers to beat him.
spoilers:
He then betrayed Rita and killed her, |
end of spoilers thus gaining the resources of Rita and combining it with the powers that Zordon created.
Even in the current storyline with his armies of Rangers and Zords, any hero teams that get a decent warning can stand a chance of resisting his forces until help comes, and the current MMPR, along with a few other friends from other teams, have been conducting hit-and-run attacks for various mission objectives that keep catching him off guard. Part of it is that Drakkon can steal Rangers' powers and imbue them onto his own soldiers, but those powers are divvied up amongst soldiers and thus are a fraction of their power. Plus, Drakkon has an arsenal purposely built to drain Rangers of their powers and their connection to the Morphin' Grid, but he's dependent on overwhelming numbers and the element of surprise to ensure he drains our heroes. But even then, the RPM Rangers and the forces of Corinth have been able to hold his forces at bay, and they're definitely not among the stronger Ranger teams, let alone Sentai teams.
A couple things to consider:
- Sentai is not without its own resources. Obviously Timeranger has time travel abilities, but in Super Hero Taisen, when the villains wanted to manipulate the Gorangers and Kamen Rider Ichigo in their prime, Gokai Red and Decade beat them to it. I say this because Drakkon has the ability to go after MMPR successors and even RPM (in their parallel world), but MMPR + Time Force have the means to counter him that way
- Despite that, if we're not counting Sentaiverse as including Kamen Rider and Gavan, then their dimensional travel abilities are severely hampered -- travel to alternate universes -- while not impossible, tend to be more because of the villains than an actual capability of most Ranger teams, despite being far more common in the Kamen Rider verse. Plus, Kyuranger is in a universe parallel to the primary Sentaiverse (though tbh, that sounds more like just lip service; there've already been ample crossovers between them and other Rangers. On the contrary, the Power Ranger-verse seems to be able to do this more frequently and more easily (since they keep pulling powers out of a hat -- esp. MMPR seasons 1-3).