McGregor's defense was not natural to boxing. Mayweather saw this and was able to walk through his guard taking little to no punishment and TKO'd him. That type of flawed defense would go horribly wrong against a slugger. I didn't expect McGregor to win so I wasn't really surprised, crossing into a totally different sport put him at a huge disadvantage, I just didn't expect him to lose as badly as he did, he barely won one round. It was also a bad idea for him to fight with 8oz gloves, I think that hurt him more.
He was only able to walk McGregor down once McGregor gassed.
He won 3-5 rounds. Showtime had him neck-and-neck with Mayweather before the TKO. He did incredibly well especially considering Mayweather abused the boxing rules to the hilt.
If you expected him to win more than 5 rounds then you've really got no clue, and if you think he "lost badly" then you also have no clue. There's a reason why boxing legends are saying the fight was competitive, and they know a bit more than you do.