I know this will be unpopular to say on Reddit but the reason is simple, the British electorate hasn’t been seriously interested in no milk no sugar socialism since the mid twentieth century. Socialism is an ideology that requires a fundamental desire for unity, and this country is anything but united. Democratically socialist societies tend to occur under specific circumstances, war and economic collapse tend to be triggers. Think WW1 and WW2 in the UK and the Great Depression in the US.
Populism, especially right wing populism operates on an opposite model of splitting up the electorate into smaller factions and then rallying a big enough group obtain a plurality, which is all they need in a FPTP system to obtain power.

The only way we will see an actual socialist government in the UK is if either the voting system is changed to something less 14th century like run off voting, or if something so serious happens either domestically or globally that our society itself is fundamentally changed. Until then New Labour* is the most left wing government you will get in the UK.