Perhaps another reason is that although they're magnificent to use, it's not possible to add goto or tracking to them as far as I'm aware. Again, I'm sure that could be solved though?
That use to be one of the reasons. However it is now possible to add PushTo on ball scopes by adapting the Celestron Starsense plate solving software for iPhone or Android that comes with some of their beginner telescopes. You can purchase the least expensive of the Starsense telescopes (about 200$) and transfer their hardware to a ball scope. You can then find many objects in the sky using only your phone. See here for details.
As for tracking there are two ways: First is to mount the ball scope on a Poncet type equatorial tracking platform which will provide about an hour of tracking at a time. But the best solution is to mount the ball scope on the tracking platform designed especially for this type of telescope which I invented back in 1996. See my website for details.
With the Starsense and tracking platform you can find objects and observe them with tracking or even do astrophotography.
I’ve always admired the Portaball as a very elegant solution - but - there is a fundamental issue - the centre of the spherical bottom defines the altitude axis and it’s inevitably very low down. The two ways to deal with this are either to make the ball significantly larger than otherwise necessary, or put a lot of ballast (lead shot) in the bottom. Either way it works out rather heavy.
I’ve seen a 12” Portaball many years ago - great to use - but it was so heavy the owner used a small gantry on the back of an SUV to lift it.
I've built half-a-dozen ball scopes, including a 20 inch f/4. When well designed they are no heavier than a dobsonian. My 20 inch can be fully operational and weigh only 80 pounds. I'm now building a 12.5 inch f/3 ball scope which will only weigh 30 pounds including 3.5 pounds of eyepiece/coma corrector in the eyepiece holder. No need for a gantry, I will be able to lift it with one hand.
The one limitation with ball scopes one must be mindful of, however, is that they are more practical in short focal ratios like f/5 or less. Longer focal ratios will indeed require a combination of a larger ball and/or more ballast behind the mirror, a problem more easily managed with a dob.
Here is a picture of one of two 8 inch f/5 I recently built. The OTAs weigh 22 pounds and the 1.5 cu foot base, which serves both as a tripod and a storage box also includes an equatorial tracking platform: