Why Did Solentjay Deny My Friendship Request?

You would have to ask that person, send them a shout.

Maybe she checked your profile and thought that the two of you didn't have anything in common or maybe it is your age. Only Solentjay know for sure.
Some adult blurters do not accept friend requests from minors. I'm sure you can understand that if you think about it.
  Not all blurters will accept friendship for different reasons. Some just have so many friends already that they can't keep up with all of them.

Well she usually say she want no more friends because she has no time to chat.

There could be any reason for this, she probably is content for the time being with her friends list. Some people just don't keep a lot of friends on here. I am sorry. Maybe after a while she will accept.

Well, she isn't accepting every request. She is monitoring your activity to see if you are on long enough and reading your profile to see how much you could tell others about yourself

Before you request someone's friendship it is a good idea to read their profile page to keep from having to ask questions such as this. The following is from Solentjay's profile page:                 Please do not request adding to friends  - I am cutting back as I do not have time to chat - thanks.