IMHO The improved efficiency of variable speed pumps and improved telemetry.
Historically pumps were either on or off and pumped a set volume. Water towers provide a small reservoir at height to ensure the mains pressure is appropriate and when the level reaches a set point the pumps come on to refill the tower / meet the instantaneous demand that was causing the tower level to drop. Pumping power to meet full demand was therefore always there unless like the American ones shown above they are huge. Their main purpose was to ensure the pressure to meet small demands e.g. overnight which are too small for it to be desirable to require the pumps to run and when the pumps did run there was sufficient volume to fill that they would run for a sensible amount of time.
Now you can sit a booster pump at the low level reservoir and a pressure gauge in the network and the pump can run slowly to meet a trickling demand and then step up at 7am when everyone gets up and has their morning shower.
The other killer is maintaining towers that in the UK are often designed to obscure their purpose, so ornate, and are now quite old. As the tower has to be able to vent air in and out as it empties/fills they also present a point at which contamination can occur, pity the poor sod who has to climb out on the top to ensure the vent is both clear and the cover in place.