Honest question -- what other reason is there? Plenty of less expensive per unit, great sounding, "dumb" multi-room speaker systems have been on the market for years. HomePod is not a game changer in that regard. Seems to me Siri IS the secret sauce here. I mean that would be its only appeal to me -- though right now it's not enough.
HomePods lack of different sized units is it's Achilles heel, though presumably Apple will gradually add to the line with smaller, less expensive models. Once their is an appropriate HomePod speaker for every major room in the house I'm very interested, but only because of Siri integration.
Tons of people bought the amazon echo because of the insanely cheap price. We will see what their retention rate is, if people will still be using Alexa months from now once the novelty wears off.
Or to put it another way, google and amazon are flooding the market with tons of cheap speakers because that’s the only way they can compete. It’s a race to the bottom, and we have already seen this play out with android phones and wearables.
My bet is that Apple will position the HomePod as a high-end accessory with some as-of-yet unknown differentiator which will play into Apple's long term product roadmap. I honestly do not think the only / primary function for stationary smart speakers is to pipe digital voice assistants.