Up work gets a lot of request everyday and they want to make sure that their costumes get the best experience. This is why they created this system. They are usually looking for people with skills that are most asked for. Web developing is one of the "higher skill" if I can say that 😃
Although I think front end is more valuable then the back end. I don't see why they will decline you that much. But you should add some front end skills as well. Some people go as far to put a completely other set of skills and after they get accepted they changed their profile.
Your profile is not bad but it could be better. Try something more specific like this:
"I an expert developer in PHP , Ruby and Python from [X Country] who loves working with people. I have an enormous satisfaction when I see ideas came to life.
[ / I am self-tough / or / Started coding / or / I graduate ] about X years ago . My focus being backend development and as the years passed by I have worked with plenty of people on both small projects as: X, Y, Z and big projects as J, K .
I specialize in X as is my favorite [ platform / or / language / or / or / i don't know/ ](here you can add RubyonRails and Django) to do X . In my 5 years of professionally website developing I also had the pleasure of experimenting with front end and design. "
As part of your profile you should also add some portfolio peaces. I'm suggesting no more then 3 but they need to be your best projects.
Also perseverance is key ! If they declined you , try again ! If they declined you again. Try again !
You can also email them an ask them why you keep getting declined. But I am suggesting not going at them like
"Why you keep declining me you $#&# !" . You could just send them an email like "Here is my profile, I keep getting declined. I really want to be accepted. How can I make my profile better so I can make that happen".