Why? Bad planning?

When I was a student, I was registered to vote in central London. The polling place was still only responsible for roughly the same number of people as some place in the countryside. There were simply more of them in the centre of a city.

Deliberately bad planning. Polling locations and times are determined by folks with a vested interest in choosing who votes for them, not in letting voters choose who holds office.

That's definitely a problem in the system. Organizing the election should be independent from political interests. Not having that independence is a far bigger problem than anything else.