This article is from the Real Ale FAQ, by with numerous contributions by Brett Laniosh others.
A: Because it keeps for many months, is easy to standardise month in month
out, and doesn't require any attention in the cellar or at point of sale.
If they arrange the gassing up correctly the beer is served with a large
head which looks like it ought to on the telly, but being air and not
liquid is an excuse for serving short measures - every drop paid for but
not served is pure profit.