To be fair to Crocodile, while I'm sure the other people he's challenged or fought could probably hit him about as well as Luffy did in Alabasta, not many of those people have quite as high of a DPS as Luffy does. One thing that's been consistent in the series from the beginning is that Luffy is crazy fast with his strikes, much faster than most people's reaction times can keep up, therefore he's got a much higher chance of actually landing a hit with his wild punching than, say, a Whitebeard who can probably cut an island in half but has a windup of about 4 seconds before his weapon actually hits.

I would also like to bring up that Crocodile had likely gotten lazy, for a lack of better words. I mean a logia user in Paradise where most people still didnt know haki was already nigh untoucable, his organisation had its claws deep in the country. All his other fights in Alabasta could quickly be resolved with a touch of his hand. He wasnt ready for someone who could stand up to him, much less a madlad like Luffy who goes all in even he has to use his own blood to hit him