My thoughts are similar.
Several factors involved. Speaking of rifle caliber NFA guns.
The fixed number of guns in the loop with an increasing nationwide pool of buyers, largely due to increased access of information, i.e. the internet, on all aspects of owning and crooked dealers to steer clear of.
This has led to more buyers taking the leap.
Surplus ammo is pretty much done, and definitely done at 'less than the cost of the bullet' prices.
Bulk domestic (and new foreign production) prices for the 7.62x51 and the other belt fed calibers are unlikely to ever be structured anywhere close to that of the 5.56 and 7.62x39.
Most belt fed guns fire from an open bolt and, in my experience, are less forgiving with head space issues when shooting reloaded cartridges that have been reloaded multiple times.
Spare bolt parts for the belt feds are not as numerous in the first place and shooting reloads with resultant case head separations is hard on them.
My take, JT