Because it gives them an excuse to pass policy that favors their donors under the guise of "electability", even though time and time again, it's been shown that triangulating yourself to be one inch less bad than the republican loses elections over and over because it demotivates your base when they vote for the candidate that will do X Y and Z, and the candidate passes republican policy instead.

Do you think the democrats didn't pass free healthcare in 2008 because they genuinely believed that there's a big population of centrists who enjoy paying money and dealing with health insurance and will vote them out if they get free healthcare? Or because they got and continue to receive billions of dollars from the industry?