The reality of the situation is that Predator is perfection when it comes to killing.
So are Metroids. They were literally designed to be the perfect killing machines. Samus made their entire race extinct. Beating Predator is nothing in comparison to wiping out a species that sucks the life out of you and doesn't stop until you die.
Everything about its anatomy, training, and equipment it carries is designed to make it a more effective killer. Samus is good, but bounty hunting is just her profession. Killing is Predator's life.
Samus has wiped out at least two species, the Metroids and the X-Parasites. Her 'profession' involves her arriving in a section of the galaxy and killing everything in her path, whether that's a space pirate, a giant brain, an alien organic energy being, a space dragon. You name it, she's killed it. The Predator hunts, Samus commits genocide.
Samus has some heavy hitting gear, but so does Predator. Plasma weapons,
Plasma weapons? How cute. Samus uses plasma weapons to do her welding. She also has weapons that can kill you while you're standing on the other side of a reinforced wall... which can be combined with aforementioned plasma beams so she can set you in fire before you even know where she is.
a bull whip that can cut through flesh and bone,
Cut through flesh and bone? That might matter if Samus was facing Predator while in her Zero suit. However, she isn't. She's in a suit with a powerful enough force field* to survive a dip in lava without taking any damage. Predator can whip Samus all he wants. He'd be lucky to scratch her suit at all.
a homing razor disc that looks to cut its target into pieces, and that's just standard equipment.
Anything stopping her from shooting it?
Predator and Samus are probably about equal in terms of technology
Not even remotely. Samus can kill everything within a 100 yard radius in about 12 different ways, including dropping a goddamn nuke on the area from her ship. Predator hunts, Samus commits genocide.
but training-wise and culturally, Predator is just too determined for Samus.
Determination doesn't matter much if you're already dead. If you're unlucky enough to be on the wrong side of Samus' arm cannon, you aren't walking away.
This is a tournament fight for Samus, but it's a fight for honor for Predator.
Samus is treating this as a job. Her job is to kill things. Predator is just one more thing on her list.
I'm giving this one to Predator and Sub-Zero, and you should too.
Is this because Samus beat Shepard last year?
*This is an assumption on my part, since getting hit reduces her energy levels, rather than health.