Some compounds just seem to naturally shift from hyphen-separated to no separation. Just one facet of the evolution of language, its spelling, and the randomnes

Some compounds just seem to naturally shift from hyphen-separated to no separation. Just one facet of the evolution of language, its spelling, and the randomness at play. E-mail isn't necessarily any more deserving of a hyphen than words like permafrost or sitcom are. Why didn't they get hyphens? Who knows.