Please do name all these other "highly skilled swordsmen" they have lying around. The best swordsmen are dead, one handed, or gone. The only person besides the Mountain known to be a good fighter is Loras, who conveniently is a Tyrell and whose family would never allow him to fight. The Crown will call whoever they think is the best fighter, before Tyrion even gets a chance to name his fighter. Would Tywin, who suspects Jaime to be worthless with a sword, allow Jaime to be called to fight for Tyrion?
Oberyn has done nothing but cry about his sister and fuck whores while in King's Landing. Well, that and confirm Joffrey was poisoned. The only logical choice for the Crown (and Cersei, the accuser) to call would be the Mountain. Bronn was the only ally Tyrion had, and as a sellsword, not exactly loyal. I could see expecting him to fight for Tyrion, but not expecting Oberyn to at least voice his enthusiasm to kill the Mountain in the trial is just not paying attention.