To be clear, this is about the Mac App Store (MAS), not the (iOS) App Store. In both cases, you're effectively paying Apple a cut of the profits in order to make your product more accessible to consumers. In the case of the iOS App Store, it's pretty obvious that the 15%/30% cut is worth it, since if your app isn't there, it isn't for sale as far as 99.9% of people are concerned (even though that's not strictly the case).
But the MAS? Its value proposition has always been questionable.
For one, purchasing patterns are drastically different between mobile and PC. Consumers typically already know what Mac apps they plan to buy, rather than browse-shopping like they do on iOS, so whether the app is in the MAS or on a website makes no difference. As such, developers don't lose much from pulling out, or, in many cases, what they lose in unit sales is more than made up in reduced overhead.
Making matters worse for the MAS, it's oftentimes the case that the version of the app sold in the MAS is both more expensive and has less features than the one sold on their website. The MAS has a number of requirements (e.g. strict sandboxing) that make certain features virtually impossible to implement, so the apps in the MAS are oftentimes missing key features found in the direct-sale versions, or they might be lagging behind by a few versions due to the app store review process that all updates need to go through. And because developers don't see much benefit from the MAS, many of them simply tack on a 30% premium for the version sold through the MAS, that way they can recoup the cost. But even in the case that the developer doesn't price it higher, there's no way to offer upgrade pricing for loyal customers, so MAS users end up paying full price for subsequent versions, rather than being able to get a discount that the developer might be offering on direct sales.
All of which is to say, the MAS is a somewhat hostile environment to both developers AND users, so it's not surprising that niche apps aimed power users (i.e. the ones most likely to know how to use a browser to find software) are seeing improved numbers after pulling out of the MAS.