Political parties that edge closer to political power tend to slowly get invaded by careerist politicians put forth by lobby groups intent on maintaining the status quo.

This is how most Labor parties end up meek allies of capital - career politicians injected into the parties are funded by, promoted by and supported by institutions that owe their allegiance to capital. It's very much the safe route into politics that ends with a cushy job (which is critical coz most political careers end in failure). These people are often the best at co-opting political machinery (e.g. getting on the right committees, swinging important but less visible votes, figuring out how to get "difficult" members ejected based upon trumped up accusations of racism).

For anyone else taking a political career route you risk ending up unsupported, overworked, villified and subsequently ejected into unemployment.

If attempts to co-opt political groups edging closer to power using internal party political machinery fail, there's always a media under the full control of the upper classes that can villify individual politicians that are a threat, which a good 1/2-2/3 of the population will usually believe. Sometimes this works hand in hand (e.g. creating a facade of "unelectability" via the media and then using that internally to nix career progression).

None of this happens in parties with zero chance of getting into power (e.g. most green parties) so ironically they often end up with the best platforms.