Real ale (cask-conditioned ale) has taste and needs careful cellaring to get the best out of it. The skills of the cellarer has an important effect on its final taste.

Lager generally means ice-cold, factory produced garbage, very much over-priced and sent to the pub in sealed metal dustbins. The cellarer has little effect on its taste since it is brewed to a lowest common denominater and offends no-one.

And yes there are some great lagers, generally brewed abroad and served in bottles. Mass produced lager is served cold to distinguish it from gnat's piss.

Much the same applies to Guinness but there are many myths about Guinness so it is hard to convince people it is no better than lager.