We are all members of society. We each have an equal right to the decision-making process for how our society is organized. But many people who want to change society are using the political party. Their tactics are to encourage voters and have their candidates run for election. But does this method really help you out? Does your candidate fulfill your hopes?
If you want to change something in society, you need to have your candidate elected. This is the option that the political party provides for you. And you're stuck to that candidate, if they're the only one who supports your platform. You'll have to support them in their other campaigns, in their other ideas, whether or not you're completely opposed to them. It's the only option of the political party, if you want to have a chance to change society.
In many elections, it is a close match between the two political parties, the Republicans and the Democrats. Your candidate wants to get more votes, which means broadening the platform. Many voters will not whole-heartedly agree with all proposals, but they'll agree with some. To make themselves look better, the candidate will make their platform less concrete, less meaningful, and less real.
By moving towards the center, politicians are making themselves look more appealing to the undecided voters. If they're elected, they won't be enacting your vision; they'll only be enacting a heavily compromised version of it.
To get the vote of enough people itself requires massive mobilization of people and organizations. You're trying to get enough people to vote for a candidate who supports your platform. If you can get enough voters, why not encourage those people to fulfill your platform instead?
For example, if you are working for labor rights and improved standards of living, form unions. Ten million workers on strike will do plenty to increase their wages. Ten million voters dropping pieces of paper into a box will do virtually nothing.
A thousand volunteers creating labor agencies will do much for increasing employment. But if those same thousand voted for some law, they'll be some minority that gets overruled. A hundred people organizing for public awareness about discrimination can change the entire social order. But that same hundred lobbying in congress would be fighting a struggle they'd never win. Those who want to protect the environment would be more effective at convincing workers to strike for ecology. At least, when compared with lobbying a bunch of senators who live off of environmentally-unfriendly companies.
The same number of voters, organized into associations, accomplish the same objective. And they can do it better! Become uninvolved with the government. Become involved with your community.