To some extent: politicians, they're just like us. Who do they want to be liked by? The people they spend time with. Who do they spend time with? Mostly not the median constituent.
Sometimes you hear about politicians who received, say, FORTY THOUSAND DOLLARS in campaign money from some industry, offered up as a corrupt explanation for votes they take. I'm not dismissing that entirely, but I also suspect it's basic peer pressure a lot of the time.
Wanna be the cool kid at the parties you go to.