It sounds like Mr Collins was only recently appointed to his parsonage by Lady Catherine, and therefore only recently acquired an income to marry on. Chapter 15 talks about Mr Collins' early and unexpected prosperity.

So Mr Collins may have met some eligible females before, but it doesn't sound like he was eligible before - not having much of an income of his own - and he does not come across as a man likely to create a good deal of passion for his bonny blue eyes.

Then JA talks about Mr Collins living in retirement. She's talkig about the effect on his character, but if he is living in retirement this implies a small parish without much social life. We do know his parish is in Kent rather than a big city like London, if it was not near a sizable town there might fail to be any unmarried young women around of a status that Mr Collins could marry (ie I don't think a woman with a good dowry would marry him, and I don't think he'd marry a woman considerably lower in social status than the gentry). His relations might just appear to him to be a good place to start.