The short answer seems to be that in return for good committee assignments, congressmen are required by their party to meet certain fundraising goals. If congressmen do not meet these goals then they may get removed from choice committees and the lack of funds will put them at a competitive campaign disadvantage. It's to the point where congressmen may spend over 2/3 of their time soliciting funds and must regularly solicit lobbyists for money.
The short answer seems to be that in return for good committee assignments, congressmen are required by their party to meet certain fundraising goals. If congressmen do not meet these goals then they may get removed from choice committees and the lack of funds will put them at a competitive campaign disadvantage. It's to the point where congressmen may spend over 2/3 of their time soliciting funds and must regularly solicit lobbyists for money.
http://www.npr.org/blogs/money/2012/04/19/150904641/why-lobb...