I think exactly what JD said. All the local FB pages were filled with HS2 reasons and they appeared to be the only choice.
They were also the only ones that bothered, visiting houses, letters, leaflets, and while some found it annoying, others seemed impressed with the effort. To the point someone posted "Sarah Green. I've only heard from you once today. U OK hun?" Lol.
Temporary and for a specific reason. Plus- it's so blue here, people often won't bother at all (particularly in a by-election) unless they positively want someone else. Complacency all round.