Why dont guys ever want relationships with me? They only see me as their 'hookup' girl!?

I do NOTHING to egg this on either! Im 21, extremely focused on work and my studies. One guy ive known for over a year..we did hook up once...and thats all he wanted afterwards and still after 6 months he doesnt want a relationship. Married men and guys with girlfriends hit on me behind their wives/ girlfriends backs...and Im not a little skank either..i dont wear revealing clothes..im very cly and proper as a 21 year old, just whitty and smart and make people laugh...nothing overly ual at all! This guy i met once at my university found me on facebook and just asked me if i wanted to get 'friendly' with him, and i told him I dont do anything with random guys, im a cly girl. I JUST WANT A REAL RELATIONSHIP FOR ONCE! Why do every guy i meet want something compeletely opposite? is it something about me that I have no idea about?! Could I be too nice which makes them beleive they could walk all over me?? Im so sick of it!