This is a great question TS and one I’ve pondered myself. For everyone who said money, I’d argue Khabib wasn’t a huge draw in either the previous Iaquinta or Barboza fights. Conor would always be the upside and draw money with whoever he chose to fight and he had other options. For example he was planning to fight RDA for the interim Welterweight title at UFC 225 and looking back now would have had a better chance of beating Woodley for the Welterweight title, who we now know was on the cliffedge of decline than Khabib. At least Woodley would have stood for him.
Secondly, he actually had lots of other options: the perpetual trilogy with Nate Diaz, rematch Aldo at catch-weight or another ill-advised foray into boxing (which might have actually made him more money and provided the same opportunity to promote his Whiskey).
Thirdly, having made all the money with Floyd he could simply have retired as the Champ Champ and left the question as to how he’d have done against Khabib forever as speculation in the minds of the fans. Rather than take the risk of being Khabibed in a humiliating Michael Johnsonesque fashion.
However, I think two things meant that he decided to fight Khabib. One was that the narrative around Khabib in 2018 (following the Iaquinta fight) was different than it is in 2021 (following the Conor, Dustin, Gaethje fights). So for everyone who is saying he wanted to challenge himself against the undefeated streak and the greatest Lightweight of all time this isn’t the case. Conor and many people viewed Khabib as having a padded record and not having fought anyone (a peaking RDA aside) of significance.
Directly after the Iaquinta fight the narrative doing the rounds was that in a five round fight, if you could survive the first two rounds, Khabib would slow and anyone with decent striking would have their chance. This was the narrative put out there by varying figures on the MMA pundit spectrum from your Brendan Schaubs, your Dan Hardy’s and even your Weasels.
Dan Hardy in particular cited the Chad Mendes fight as a prime example of how Conor could weather the storm and then knock Khabib out in the later roundS. Unfortunately for Conor the MMA math turned out differently.
Rather than KI (Khabib vs Iaquinta) x CM (Conor vs Mendes) = Conor late round knock out.
It turned out to be:
KI (Khabib vs. Iaquinta) x CD2 (Conor vs Diaz 2) = Conor tapping out
As we know now, two rounds of grappling meant Conor had no power in the later rounds and so when Khabib did slow in the third Conor couldn’t hurt him. Khabib then picked up the pace in the fourth, just as he did in the fifth round of the Iaquinta fight, and tapped Conor out.
As well as Conor and lots of the MA media getting the narrative wrong – again it’s retrospectively incorrect to say that no one with any knowledge of MMA predicted Conor would win – you also had Conor’s state of mind.
His impulsive behaviour in attacking the bus and various out of the ring incidents showed that he was acting in a manner where he didn’t care about the consequences. And his behaviour in the Press Conference was in the words of the Weasel like ‘a king gone mad.’ He didn’t care or think about the consequences of losing to Khabib. Even though we now can see it represent a seismic shift in the MMA landscape. No longer would Conor be bigger than the UFC or the sport of MMA. People who argue that Conor did the least badly against Khabib, or celebrate that he won one round against him illustrate how far people’s expectations around Conor have fallen and how high Khabib’s credentials have risen since 2018, compared to Khabib who has everything.