The UK and the US both suffer from having political systems dominated by two parties. There are a number of reasons why this doesn’t lead to good democratic outcomes.
Firstly it gives us something adversarial in nature. Us versus them. Polarised tribal politics with little room for co-operation. We’d be better served by compromise.
Secondly, most issues have more than two sides to them. If you aren’t represented by the two sides in an argument, you’re stuck. More parties means more breadth, depth and diversity.
Thirdly, if you then feel unrepresented you may well see no point voting. This is part of a narrowing down, as the two parties go after the people who do vote, they may move closer together, representing fewer people.
Fourthly, it is difficult to shift back and forth between two parties. If they are at all different and you agreed with one, the odds of that changing at the next election aren’t that high. Democracy works better when we’re offered a range of options that might be relevant to us and we get to decide what we think is best.
When there’s a bit more diversity, there’s more room to look at the individual qualities of candidates. How honourable they are, whether they keep their election promises, how they treat people, what kind of results they get and so forth. If you’re stuck with two viable candidates and one of them is unthinkably awful, you may feel moved to vote for someone who is simply less bad. More options tend to improve quality.
Unfortunately, the two party system serves the people who are in the two parties – the only people with any real power to change that system and open it up. So, little wonder that they don’t, usually.