In the short span of time that short films allow, there isn't nearly enough time to fully develop the main characters. Sometimes short films succeed, but it's still not the same as a feature length film. On average, short films are approximately fifty minutes or less. A feature runs for approximately 90-100 minutes. That's TWO TIMES the amount of time to understand and fully develop protagonists. Audiences want characters that they can feel attached to, connected with. It's incredibly difficult to do that within a short film. Although some of them may succeed, a lot will not. It's not worth scanning through a million short films just to find one that does it right.
Often times, we get to understand and know characters even better than we would in a feature. We get a specific snippet of life versus a general oversweep.
[P1] Characters are not as well-developed in short films. [P2] Features are better at featuring well-rounded protagonists.