Because it was the biggest Striker vs grappler fight of the time. Two of the very top guys, the one a world class grappler vs the other, a world champ kickboxer. These fights are always interesting. Kind of like Khabib vs Conor.
It is considered great because Fedor was actually able to somehow compete with Crocop on the feet, and found a good way of winning the fight. Crocop also did a great job on the ground vs one of the best mma grapplers of all time. When we also know Fedor went into the fight with a fractured hand, it is pretty impressive.
It was a real thriller, as everyone knew that both guys could finish the fight at any time, either by submission or ko.