Because it simply does not say they washed them during the tribulation = Simples, as they come OUT of it AND wash their robes, the truth was revealed, hence they had to wash off the lies, with the True Lambs blood.
Hence, it could very well be that during it, that they were Not prepared to let go of their 3 sun gods worship, the reason for the tribulation, to knock some sense into them lol. But because they clung to the basics, that there was a Father and Son, despite their silly creations of them, that perhaps, this still allowed them to get a foot in the door ????????
