Alright so lets start with the whole "nice guys" thing. There's two points to make about so called "nice guys"-
1. Most of these self-proclaimed nice guys aren't nice guys at all. They're just as selfish, hateful, greedy and insensitive as any other guy they're just quieter or more of a push over than the other guys so they don't get the girl. "Nice Guy" is a term for one of two types of people- a push over who never makes a move (ever) and never goes out of his way to express his interest (for whatever reason) or a guy who's a douche bag but isn't as good at hiding it as that other ass hole so he just bitches and moans when he doesn't get the girl. I would never insult any of my male friends by calling them "nice guys". They're good guys, good people, great boyfriends.
2. "Nice guys" also identify as nice guys because that's all they have to offer. "Why won't she date me, I"m nice!" well, yeah, but what else to you bring to the table? Go check out that article I wrote a while ago and you can read all about why just being a "nice guy" isn't good enough.
Now, who you might see as a player could very well be a good person. He's your competition, all you're going to see are his faults, his mistakes. Also, lets be real, you call him a player, do you even know him on a personal level? Are you two best friends? No? Well then who are you to judge his character? You can only see the things he's done in the past, you don't know him, you don't know how he feels about her, you don't know if he'll change, or even if what you've seen is his usual behavior.
Also, she probably picked that "player" because he'll make for a better partner than you. Now, notice how I didn't say boyfriend, or relationship- I said partner. The first thing you want is someone you can spend time with who will make you laugh, make you smile, share common interests with you. Now these "players" tend to be more outgoing, tend to share their interests and interact with girls more than most "nice guys". So we get to know them better, we end up liking them better, we see more of their positives and we see that they aren't necessarily just a "player".
He probably has shown her that he has more to offer, simple as that. They probably have more things in common and get along better. Just because she didn't pick you doesn't make it her fault or his fault. You two clearly weren't compatible for various reasons, so go out and find a girl you do work well with and stop pointing fingers. All it does when you talk like that is make us roll our eyes and pretend to get a phone call so we can leave the conversation.
You sound very immature and childish right now, try to fix that so next time, when you see a girl you like, you aren't hung up on these preconceptions and you'll actually have a fighting chance.