Why place humidifer on cold air return
The biggest reason is probably &quot;because that's where it fits.&quot;
It's true that warmer air can carry more grains of water than colder air can. That's a good physics-based reason for locating the humidifier on the supply air duct.
But lots of homes have an air conditioning evaporator sitting atop the furnace. Furnaces don't often get installed in a premium room; instead they end up stuffed in a closet somewhere that's lucky to have an 8 foot ceiling. Then the supply ducts have to remain below the floor joists, and the ducts are about a foot deep, and next thing you know there are only 7 feet of vertical height to fit the furnace and A/C into. There often isn't vertical space to accommodate a humidifier. Oh, and there are one or two ducts for the exhaust flue and maybe intake air too that come down in front of the furnace. And so on..
A return air duct, on the other hand, is often a large blank canvas. There is lots of unobstructed surface where the humidifier can be mounted. It might become routine to some installers to place a humidifier there &quot;because that's how we always do it&quot; even if there were room on the supply duct on a particular job.
A humidifier should be able to get the job done regardless of which duct it's installed on. If yours isn't, maybe you could ask a question about that.