The single biggest reason is the way the power structure is governed by the parties in congress.
Almost without exception, the leadership comes from congressional districts (in the House) and states (in the Senate) where local demographics and/or gerrymandering guarantee what are essentially lifetime sinecures.
Because their seats are safe, and because both houses of congress promote leadership in large part based on seniority, the leadership eventually becomes entirely divorced from their consituents – whom they take for granted – and much more attached to the special interests who hand them bags of boodle, which they then pass on to select fellow legislators in return for cementing their control of the House and Senate.
Some point to the term limits in my own state of California, and claim that it demonstrates that inexperienced legislators only become tools of more knowledgable lobbyists for special interests. But the federal leadership already is bought and paid for by the special interests that batten upon them, and are battened upon in return.
That is the system we have today and, short of simply limiting the longevity of these creatures, I can see no other way of reforming it. While I support Mark Levin’s effort to summon a convention of the states to reform the Constitution, I think a simple term limit Constitutional amendment would stand a much better chance of passing.