Why the net became dangerous
Serve-and-volley tennis depended on opponents being unable to reliably pass a player standing at the net, and for a long stretch of the sport's history that was a fair assumption. As racket frames grew stiffer and string technology improved players' ability to generate topspin and pace simultaneously, passing shots became dramatically more reliable from almost any position on the baseline. Committing to the net stopped being a moderate risk with a strong reward and became a high risk with a shrinking one, and players adjusted accordingly, mostly by staying back.
A tactic pushed to the margins
For a long period this pushed net approaches to the margins of the sport, used mainly to end points that were already effectively won rather than as a way of creating opportunities. Baseline rallies became the sport's dominant currency almost everywhere, on every surface, and players who tried to serve-and-volley as a full-time strategy became rare specialists rather than a recognisable style among many.
Selective net play's comeback
What has reappeared recently is not the old full-time serve-and-volley approach but a more selective version, where players move forward deliberately after producing a specific weakness in a rally, such as a short or defensive reply, rather than as a first-strike tactic off every serve. This version treats the net as a reward for already winning the point mechanically, which suits an era of reliable passing shots far better than the older, more speculative version ever could.