In my opinion, it's because yuri or lesbianism was known for the subtlety. It's like a frail flower in the middle of storm. Same-sex relationship was not largely accepted, but two girls holding hands in public is socially acceptable. It was a little act of confirming their love in public without being overtly obvious. But, yes, I don't actually know when or why that symbolism turns lewd. Maybe it's because the little known fact that holding hands is the third base of lesbian sexual act, and as we all know that holding hands usually leads to Luminous, which is the most depraved lesbian sex act known to humanity.