I'm no expert but from what I've read the process to make Rosin doesn't scale as well as a hydrocarbon process for mass production.
Like
@psychonaut said, the folk who produce for dispensaries have large closed loop setups that are closed loop so the solvent can be reused. There is usually also a greater level of extraction meaning less wastage and higher returns.
Then there is the fact that you have to use pressure and heat to produce rosin. The heat can somewhat degrade the terpenes during the extraction process if the extractor doesn't pay attention or is new to the process, this would result in a lower quality end product.
More recent CO2 extractions can also produce less viscous results which works well in conjunction with cartridge and atomiser based delivery mechanisms.
With rosin you also can end up with small particles of plant matter in the resulting extract which can clog up or burn onto atomisers, less of an issue with nails though.
For small home use Rosin doesn't require complex closed loop setups or dangerous open extraction methods. Nor is there a requirement to purge the end product.
Some of the newer tech can even separate out the different terps and actives meaning the larger companies can remix them creating very specific products to more consistent standards. Which is important if you are trying to define a brand and product line.
(Is that quote sexist now?) 
