I think the size of a company and its focus all have a bearing.
Mainland and Ohana, although apparently made in the same Chinese mass production location (please correct me if that isn't the case) are very much ukulele-focused. Their quality control is reputed to be good.
Brands such as Leader, Vintage, Westfield and Stagg buy and badge guitars, ukes, mandolins as well as often also doing cases and accessories (even amplifiers) for all these and more. They are larger operations whose appeal is in offering a wide variety of budget products to people who are used to buying cheaper instruments - and who presumably accept that one does not get great quality at very low prices.
People like Ohana, Mainland, Kala etc specialise. They occupy a middle ground where the basic instruments may be made in the same locations as the bulk-shifters, but operations are at such a scale that quality control is still important. Reputation still plays a part.
Stagg products turn up in music stores all over. To an extent they will sell simply because of their visibility. Ukes in general are not as widely distributed. People will go hunting, they will research, ask questions etc etc. Feedback from places such as UU will have a larger impact. Therefore reputation will have a larger effect on sales.
I would expect that the publicity MGM gets here has done a huge amount for his business. He ensures that continues by feeding back into the positive loop by providing good service, which is then reported here etc etc. The uke brand owners will see the same thing.
The basic instruments are probably sound enough, whichever of the cookie cutter brands one chooses, but quality control can, and in my experience does, make a huge difference. My Stagg concert, for example, should never have left the factory with the bridge crooked by about 3mm as it obviously affected intonation. Other details such as poorly cut nuts etc should be caught by decent QC. Once I fixed my Stagg's bridge and made a decent nut for it it became a very nice uke. Out of the box though? Sub-standard. Not in tune and a poor player.
If a rebranding distributor takes any crap that the factory produces there is little incentive for the factory to make any reall effort. If the brand owner refuses to accept sub-standard QC the factory has the choice of shaping up or losing the order.
Other variables between brands will be things such as the hardware and strings selected for fitting as standard.
I don't think it's really a question of bad craftsmanship. I'd say that quality control is the biggest thing. The more brands that keep a close eye on it, and that refuse to take poorly finished product, the better the overall level of quality will be.
In the last 10 years, the quality of Chinese and Korean instruments in general has improved by a fantastic degree. I have had some great Chinese mandolins (Eastman and J Bovier) and I have friends who own a couple of real great-value Chinese fiddles (Eastman). My own new Kala ukulele seems to be very well made and finished.
Some of these cheaper Chinese ukuleles are terrific. Don't dismiss them all in a bunch.
Quality control is a huge factor. IMHO, It's what separates the good from the crap, even more than a different design or a different factory.
YMMV, of course.