It's called character development. Finn now has something to prove to others and himself in the next movie. It also shows the casualties of battle and the sacrifice Finn made to protect Rey. Once that point was made his purpose/story for this specific movie was over. So him being in a coma is fine and adds uncertainty for the audience.
1. Kylo took a bow caster shot straight to the abdomen. He's basically dying from that point on. The movie even goes out of it way to show Kylo in pain and bleeding out.
2. Kylo is not as powerful as he leads people to believe and he knows it. He's struggling with his own identity and not being able to live up to expectations.
3. Kylo is very unstable mentally and physically during the fight vs Rey and Finn. With Finn he let his guard down. With Rey he underestimated how much of the force she possessed. Both things are typical of someone who practices the dark side of the force. He is not a Sith nor a Jedi and probably turned to the darkside as a padawan, he's just a practitioner of the force now.
Don't make the mistake of comparing all Jedi to the Best of the Best. The Prequels and the Originals dealt with the creme of the crop, Obi-wan, Yoda ,Luke, Anakin, Mace Windu, basically legends of the Jedi Order. But take two seconds to look at the Jedi in Clone Wars at the stadium or how easily they were taking out by order 66. Jedi aren't untouchable gods and many non force users had the ability to kill them which is no secret.
So bare in mind all of Kylo's disadvantages/shortcomings and the fact that he wasn't fully trained into the equation and its not so far fetched that Rey and Finn didn't get obliterated.
Also, Snoke instructed Ren to bring Rey to him... so he had to keep her alive regardless. Not to mention, Ren told Rey that he could train her in the ways of the force... so he never had intention to kill her at all. And he may had assumed that Finn was dead considering that he DID sliced his back up with a light saber.