While I don't have specific experince with Forum Spark as a platform, I do have lots of experience using other free platforms, specifically Simple Machines Forum.
I switched from using SMF to XenForo (XF) after nearly two decades of comfortable experience becuase of the lack of developemental growth SMF was experiencing. It was mostly stagnant. Sure, there were security fixes and still a fairly active member base writing expansions for it, but I wanted more. XF gave me the little features that I was craving, especially with a sleek user interface. It's not perfect, but it's much closer than what SMF could give me.
So, the reason why the free platforms don't get chosen is because the lack of features that are in demand. And it's usually the little things that are often overlooked that users truly want.
For example, I'm a member on several other forums, and the software is quite different among all those sites. Working with my own site, I've become comfy with the features that I've got implemented. The range of text manipulation options, buttons that expose exactly what I can do to that text. Not everybody knows MarkDown syntax, or remembers all of it. Sometimes, I want to add a GIF, just to add emphasis to what I'm writing about, or I'm feeling ... extra spicy. This is just one package of features that I see could be missing from the Forum Spark platform, but I'm not sure, because I haven't looked under the hood, and I probably won't be motivated to do so, because I'm quite happy with my experience with XF.
And remember, in my opinion, a forum isn't a very good place to generate an income stream. People visit forums for information, and to have a conversation about a specific set of topics. As a user of several forums, most of the time there are advertisements on a site, I usually ignore them, and if they become too obnoxiously in my face, well, there's lot of other places I can go explore on the Internet. The best way to make money on a forum is by granting additional topic spaces and by upgrading people's user experiences. At least, that's what I'm currently thinking. But then again, I don't operate my own forum with the intention of making any money. I just want a community of like-minded people to hang out with.
Anyway, I'm only a user here, and this is my opnion.