Because people are too embarrassed to ask for them or want more sophisticated ones, ribbed, flavoured, coloured etc etc. Or people just don't have time to go to a clinic.
OUr local surgery even has a condom club, you join and they give you a months supply each month for free in a bag you go and pick them up at the start of the month. I don't know about sheffield uni but when I was in aber uni they used to have them in big baskets in student services and you just helped yourself.