Lots of reasons.
He intended to call but changed his mind. He never intended to call but was being polite. Maybe he lost your number. (It happens--it happened to me. Someone stole my pants and the number was in my pocket. Long story.) Or maybe the guy got hit by a bus.
Whatever the reason, rejection sucks but life goes on.
Thanks for the question.
I actually asked this of a 43 year old single platonic guy friend a while back when he was summarizing his last couple of relationships (mistakingly thinking I cared).
He told me about this girl who was "smokin' hot" (I know...feel free to eyeroll) and he was all excited that she gave him her number when he approached her at this lunch place.
I asked if they had a date yet.."no are you kidding?" he says. "She's so outta my league!" And he went on to tell me how he assumed she was probably too high maintenance and demanding...and she looked like trouble. So I asked him, then "why did you ask for her number, you moron?"
He replied...well...I just wanted to see if she'd blow me off or not...but she didn't...cool..". It was an ego boost for him. I despise these games that guys play. I wanted to smack him, but just so you know...it may not be YOU. In fact, it probably isn't your fault. You're too damn hot for those little boys...you need a real man. I know he's out there for you.
Good luck
My guess is that he was drunk when he asked and then lost courage when he sobered up. Story of my life.