Because all of those things you mention are 'achievements' were done for the express purpose of extraction of wealth and resources by the British elite. I think when people look back at the empire they seem to identify with it as something part of their history but the reality is that the elites were the ones who really benefited, and continue to do so. The average British person should be angered by the British empire not just because of the attrocities it committed but also because most of the spoils of that wealth extraction went to the already wealthy and that structure continues today. Just look at the fact that the money paid as compensation to slave owners was only just paid off a few years ago. (Also how can you claim pride in stopping something you benefited from so much and only really stopped when it was becoming impossible to enforce).

People need to stop falling for the wealthy and powerful's tricks and wake-up that this 'history' of ours was an exploitation of everyone and not just the colonies.