I know your question is rhetorical but the answer can only be because they are spineless cowards, as are Trump’s sycophants in his administration, huge sections of the media and indeed many on this side of the pond too.
I see the penny is gradually dropping with some right wing politicians here, who would have gone all in behind Trump just weeks ago, now they’ve finally realised that the British public wants no part in this. If only there had been clues that an obviously illegal war started by a dangerous madman and his cronies wouldn’t go very well.