That isn't representing their constituents, though -that is representing themselves, and their party (edit: or more accurately, their faction in their party).
Of course they're representing their constituents.
We live in a parliamentary democracy, not one where every decision is made by the general public. Anybody with an ounce of sense can see Corbyn is unelectable and would lose a massive number of centre ground voters in the next election.
Why on earth should we bother having somebody lead for their values if they will never have a chance at implementing them ?
Sometimes the general population don't actually know what's best.