It's not the weight of the bike. It's the risk of a "stupid light"[1] component failing when you're at the front of the peloton and taking out you and most of the riders behind you.

[1] A component which has had its strength and durability compromised in the interest of saving weight.

That's not the concern. They were worried about manufacturers producing bikes that weren't structurally sound in a race to produce the absolute lightest bike. Imagine frames failing during descents.

There's an argument to be made that the technology has improved, and that the limit could be lowered.