Digital Mysticism

It's a cantrip because it's the kind of all-day-every-day spell that you'd want on tap. The kind of spell that's burned into your bones; that defines who you are as a mage. Do you have flames at your fingertips with firebolt or produce flame? Are you a showman, always casting prestidigitation and minor illusion? Or are you that one mage who always has perfectly conditioned hair and bars of soap to hand out whenever the party comes across a freshwater creek?

I can't imagine in what context you'd need to care about a volume of suds... Maybe to make a smokescreen, or host some kind of foam party?? In any case, I would think the time to turn soap into suds would be the bigger issue than volume. Turning a pound of soap into suds by hand would take several minutes, right? If you've got 10 minutes to get something set up (i.e. the time you'd need for a ritual spell), you can probably handwave the volume as being 'enough to do whatver the player wants'. And if you've got a machine to do it faster, I think we're getting outside the realm of what the written rules can be expected to handle.