Because entry level res life positions involve night call and living in the halls. People can really only do that for 2-3 years before it starts to get old. At a small residential college, to be promoted up from that position, you need a Director to retire or leave, which happens a lot less often in small residential colleges for a variety of reasons. For that reason, entry level res life positions are places where you gain experience. At bigger schools, there might be more places for you to advance. At small residential colleges, not so much.
I'm having a hard time understanding why someone who has no desire to be on the academic side of a university would want to be promoted out of a position that pays you $30,000 a year and all housing and bills are covered. Explain that one.