They use this money to fund campaigns against their weakest enemies and in support the members of their caucus who are endangered. They use this money to maintain their power, the power of the majority. Of all the appeals for money the most effective are the so-called committees to maintain or attain the majority of a legislative body, any legislative body. There are few atheists in foxholes. There are fewer campaign finance reformers in legislative leadership positions.
Money is the currency with which they obtain and maintain power.
The fact that they do this so successfully creates the third, most obdurate barrier to change. The legislators themselves are also a barrier to reform. While there are courageous exceptions, most incumbents are less than enthusiastic about changing the system. It is not an accident that most of the incumbent reformers are in the minority political party. They have little to lose and maybe a great deal to gain by changing the system.