Because she cannot, in any sense, hurt him. But she can score hits and dodge around a lot.
... then again, 10 8 minute rounds is almost an hour and a half (and way longer than any MMA fight. 10 3 minute rounds is a full fight). Even she can't dodge that long. So Thanos wins.
Do you not realize that Thanos is a massively powerful beatstick while Cass is a human with some unusual chemistry and brain? They are not remotely in the same league.
I'll have to look back, but I'm pretty sure her original solo series showed her training for some hours at high intensity... And didn't her match with Shadow Thief last awhile?
And I can see high end streets lasting a few hours, considering comic book "non powered" elites are basically super human...
But yeah, no way Cass is hurting Thanos, let alone knocking him out.
Yea, she can train for hours and hours against dummies, and her fight with Shadow Thief lasted awhile, but that's probably '3-5 minutes' awhile.
Her fights with Shiva and Batman weren't short either, but still likely clock 10-15.
Cassandra can keep up exertion for a long time, but she doesn't have to be exausted, just slowed down enough to be winged once.
And while comic people can somehow roll with glancing hits from high-strength people and not die, they do get hurt by that and where one happened, more'd follow.
Oh, he doesn't have to ever notice, he can focus entirely on punching her and ignore her attacks entirely.
Are you not getting this 'she can't hurt him' thing?
Getting a ton of hits is a waste of time and energy. All she needs is a few to score points, then focus everything else on dodging and conserving strength because she cannot ever hurt him.
Ah, but you know how it works in comics. A somewhat-superhuman superagile character can often be harder to connect with than someone who has straitforward superspeed.
Especially if they're the type that only uses said speed against speedsters.
Yeah, if you're like, Wolverine, but it is one of Wolverine's established powers that he can hit any character no matter their speed by sheer force of popularity.
But regardless, Thanos doesn't need to hit her to win. He could clap his hands, punch in her general direction, or start masturbating, all capable of providing a powerful enough shockwave to kill her.
Cassandra's pretty good in that area too, being a Bat and a super martial artist.
Remember, she's pulled it on kryptonians on multiple occasions.
I do agree a thunderclap would do (I will note that comics never takes into consideration the shockwave of an ordinary miss, only something that's purposefully done to make a shockwave).