Because I'm completely and utterly torn.
I'll try to do this without party colours to try to get some objectivity.
I live in an incredibly safe seat (over 10k majority in 2010 and held by this particular party since 1880). Present MP is standing down at the Election and an unknown in his place.
I absolutely cannot bring myself to vote for him due to how I feel about the party in general on a national level. There are another 7 candidates standing in my constituency. Three of those only got votes into the hundreds in 2010 and knowing the demographic where I live I expect it to be similar this time. Another two are complete newbies and independent.
The remaining two - the candidate I really relate to and would really like to be my MP only got a few thousand votes last time - less than 10% of votes cast. The remaining candidate got over 30% of the votes cast but still over 10k fewer votes overall. On a national level I'm not as keen on their policies as my first choice but I prefer it over the party currently holding the seat.
So what would you do? Vote for the party you really want to vote for - with the little hope of them getting in, or vote for the party that's the least worst even though they probably won't get in but are a little bit more of a threat?