Etrigan is physically on par with beings like Lobo. He can take everything the Rider can dish out for quite some time.
On the other hand, Ghost Rider has been shown to be completely vulnerable to hellfire (it's one of only two things that seems to cause lasting damage to him). Etrigan breathes hellfire.
It's like putting a vampire up against a guy who breathes crosses and poops garlic. Of course Etrigan wins.