Because it’s underhand. Tactically withdrawing candidates and plotting together despite allegedly being separate parties to deprive the country of representation is untrustworthy behaviour.
Some people (in fact many) would’ve called a Corbyn victory an ‘extremist government’, would they have had the right to block his election by whittling alternate candidates down to 1 in all of the Labour constituencies?
If the country doesn’t want right wing politics they have nothing to worry about. But they do, so they did, and forced a shady result on them.
If people such as yourself are so certain that using any measures to keep right wing parties out is justified, why bother with elections at all? Let’s just have a mandatory left wing coalition and leave it there. It means you don’t believe in democracy and what does that make you?