> Greentext

Think about it on the constituency level, say you have a constituency with 100,000 people. One year, 60k people vote for party A and 40k for party B so party A wins. But during during the years before the next election people become disenfranchised with party A so they start voting for party C who they like more. In the following election, party A gets 30k votes, party B gets 40k votes again and party C gets 30k votes. Because FPTP is a "winner takes all" system, party B is now takes that constituency which is the the party A and C voters dislike most, even though party B got less votes than those other two. This is called the spoiler effect. When this is happening all over a country, sure maybe some constituencies will flip but for each that does like 30 will have the vote split leading to a probable landslide victory for party B.

Sure in your country, your vote was also "wasted" if your party of choice never entered parliament I suppose (although if you get to choose multiple parties in order of preference where it defaults to your second if the first doesn't get enough votes then it isn't wasted) but the ecosystem will be much more favourable to new parties growing because the way the voting system works makes it actually possible for them to do so. So the vote isn't wasted like it is in FPTP.

CGP Grey has some great videos about FPTP on his channel if you're interested in a better explanation that I can provide.