Because game theory. In a winner takes all system, it basically always devolves to two parties because that's the stable equilibrium.
Say we have the Winner Party, the Wannabe Party, and the Slug Party. The Winner Party has support from 45% of the populace, the Wannabe Party has support from 40% of the populace, and the Slug Party has support from 15% of the populace. In an election where everyone votes for who they want most, the Winner Party wins.
But maybe the people supporting the Slug Party realise that the Slug Party has no chance of winning. They decide that actually, while it's not their first choice, they'd rather see the Wannabe Party in power rather than the Winner Party. So they all vote Wannabe instead, and now the Wannabe Party has 55%.
Strictly, it's that any party outside the top two is completely dominated. Your vote for a third party is likely to be completely wasted in that there's no realistic outcome in which it would have had an influence. However, you can use your vote to influence which of the lesser of two evils you get by voting one of the two big parties instead, and your vote is likely to be at least relevant.
See the last US election, where a lot of people didn't particularly like either candidate but voting the lesser of the two evils was the most influential thing that they could do with their vote.
It's why a lot of modern democracies, including the Netherlands I believe, have moved away from First Past the Post systems to proportional representation. That's why you have more than two political parties. When everyone gets a slice of the power proportional to the votes they received, it becomes sensible to vote for your true preference.