Orks could easily fall to Chaos. They have all the right interests. It's just that most Orks are more interested in "being an Ork" than they are in taking skulls, or tricking their enemies, or even going fast. And Gork and Mork are the gods of "being an Ork". Worship a Chaos god and you're getting kinda weird for most Orks.
For example, rednecks like football, they like their guns, they like drinkin' beer, they like pickup trucks. They like church on Sundays. They like huntin', they like country music, they like women, and Elvis, and America. Those are 10 things rednecks like. Some rednecks like some things more than others, but every true redneck likes those 10 things. Now if somebody lets his beer drinkin' get in the way of his church goin', there's a problem. He doesn't fit in quite right with society. Letting your church goin' interfere with your football watchin' (like say, goin' on Sunday night when the game is on) and you're not really a redneck. A redneck who don't like football or Elvis? Well... there's somethin' wrong with you. People will look at you funny. A redneck who loves his guns so much that he doesn't take care of his truck, or lets his season tickets to Alabama football go? Well that's just obsessive.
Same thing with Orks. Orks are British Space Rednecks. They love Ork things. Obsess over one particular thing too much (the way you will if you follow a Chaos god), and society will look at you funny. "Gragnar didn't come down to watch the grots hit each other in the head with hammers. He's up there polishing those skulls." "Graak, you're polishing skulls too." "Yeah but I came down to watch the grot fights." While there are certainly Ork kults that follow different aspects of being an Ork (and thus probably most susceptible to Chaos worship), most Orks seem to be focused on "being Orks".