This is a speculative answer. It is nothing more than the explanation I find most reasonable.
The transition from tennis as a 'gentleman's activity' into a full-fledged sport was not entirely smooth and this is an example. There is a great difference between outmaneuvering your opponent from the back of the court, and aggressively sprinting in your long white pants to pounce on the ball before it bloody lands. It's a completely different dynamism, and I'm guessing that most people did not find it particularly endearing or worthy of admiration.
Maybe a not-too-bad example is playing football (soccer) with friends. It's not exactly fun if some guy starts being extremely physical and technical since even though he's not breaking any rules, you feel he's missing the "spirit" of the game. Really, he's just making fuller use of certain aspects of the rules than everyone else, and that's bound to piss people off.
Another example is recreational table tennis. Suppose some guy suddenly switches to a high-quality racket that enables him to create vicious spins. I somehow doubt his opposition would find this experience positive. The easiest thing to do would be to say it's unfair, or unsportsmanlike - much easier than mastering spins yourself.
As the sport of tennis became more competitive and people strove to master all of its aspects, it's natural volleying became less frowned upon and eventually more appreciated. As you probably know, volleying went on to become so central to the game, that people who didn't volley were looked down on as incomplete. Recall the initial reception of Borg's style - it was not all joy at all.