While tentative gates are known, the gate allocated could change late, for example. the previous aircraft has not pushed back and your aircraft is already on the ground so you get allocated another gate rather than sitting 20 minutes waiting for another plane to push back. With the ability to turn around an A320/A31 in 45/50 minutes, the gate is often not confirmed till very late.

there is also a suggestion that they would prefer you spend money in the bar or the shops which you will not be doing if you are at a gate, however I am sure that is not the case.

the late display of gates was not the issue here!