It is simply that the larger swept-wing jet transports have proven, in icing certification trials, that anti-icing for the tail surfaces was not needed.
The 707 had tail anti-icing, and believe it or not, some operators disconnected this feature, with the complete approval of regulatory authorities.
Lets see....I can offhand remember only about half a dozen times I have switched on airframe anti-ice in a large jet.
Engine anti-ice?
Yes, absolutely.
Airframe...not much.