There is a good reason. The old version was too much of a one-item wonder. It was a large enough chunk of armor by itself to pretty much force an ADC into Last Whisper (granted, LW was a normal part of builds then, anyhow). And since the damage was entirely dependent on incoming damage, you could just pick one up as your defensive item if they had a strong ADC.
While it's not the benchmark for balance, I do feel like in many cases ARAM provides a good place to see use and abuse cases for various things. I've played many of these where the main damage from the other team is physical. If your whole team were to just build thornmails, you would most likely turn the game around and win, because the other team would at best have enough lifesteal to nullify the damage return, or else they would just kill themselves on the return damage over the course of the teamfight. And I'm not talking only the tanks buying it. I would pause my build on any mages just to pick up a thornmail, too. It was extremely effective.
The new passive lets the buyer determine how much they want to scale thornmail, rather than leaving it entirely up to the incoming damage. It also means that thornmail can't just be your one-stop-shop for armor and countering lifesteal. Conceptually, this is better design.
But it definitely feels weak right now. Rather than reverting it back entirely, It's possible that it just needs a numbers tweak to 30 or 35% bonus armor. Or, I could see playing some more with how the damage return scales. Maybe instead of having the flat value, the scaling could be a % of incoming damage plus a % of your bonus armor - > 15% of incoming damage plus 25% of bonus armor, tune numbers as needed. Or a slightly more complicated option could be that it returns a % of the incoming damage, and that % value scales with your bonus armor - > 15% (plus 5% per 100 bonus armor) of incoming damage returned.