Yes the "3 year wait" for them to mature is a factor. So is the fact they are seasonal layers & lay an egg only every other day and frequently take breaks in between clutches. A peahen allowed to go broody naturally will go broody on 6-8 eggs at the most.. all of which creates a more limited supply.
A great many people have difficulty getting very good hatch rates on peafowl eggs, they ARE harder to successfully incubate than chicken eggs.
IMO due to this and other reasons, their price tag isn't really pricey. Sure, some are overcharging but even at 'reasonable prices', there just isn't overall that much profit after factoring in the age, total number of eggs etc.
One good thing is, at least for me, peafowl eat very little feed. Food lasts 2x longer in a pen of peafowl than in a pen of standard size chickens of the same number.
Seeing them around the yard is priceless though!