Companies visit colleges considering some parameters among which the Alumni status is one of the factor.

The colleges having alumnus who are focused and the quality of their knowledge is high and they have devoted to a growth in the company they were recruited. These criteria are collected and the companies rates the colleges such that they would get engineer with well knowledge.

So these can be some of the factors or may not be. But in general these are considered by companies those who visit campus placement.

All the best for your future endeavors!