People just don't have strong feelings about many of the candidates.
The maximum number of candidates for a constituency at the last election was, I think, 24; that's the longest ballot paper. There was another with only five candidates chasing for three seats. Generally speaking, voters just wouldn't know a lot about all the candidates. They would be reasonably well informed about the ones who mattered to them, the ones from their favourite party, the ones from around their part of the constituency or anyone else with a high profile, but they wouldn't see any need to take the time and trouble to find out about all 24 and learn about them. Some voters do—about one in 10, as my colleague says—but for most voters that's not necessary.