They haven’t said officially, and I’m just guessing here, but it does seem that since the release of the second generation hub in the fall of 2015 most of the development effort has gone into “solutions” rather than the specialty smart apps. These include the official smart lighting feature, smart home monitor, significantly expanded features in routines, and the new scenes. Between these four options you can now do many of the things that the individual specialty smartapps used to do.
Just as one example, when they added the ability to trigger a routine from a switch coming on last year, they eliminated the need for a bunch of specialty apps.
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https://support.smartthings.com/hc/en-us/articles/206370273-How-to-create-alerts-with-Smart-Home-Monitor
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https://support.smartthings.com/hc/en-us/articles/205380034-Routines
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https://support.smartthings.com/hc/en-us/articles/115004258063-Scenes
And of course for those with a strong technical background, the unofficial webcore (which replaced core) is essentially a scripting language for SmartThings with the ability to do much more complex stacked conditionals.
You put all of that together, and we rarely see community developed smartapps anymore, while there used to be several a week.
There are a few specialty issues where they come up, usually to add a feature which is inexplicably missing from the solutions such as the ability to group dimmers or motion sensors, or the ability to add an exit delay to smart home monitor. Or more advanced lock management.
But pretty much all of the general stuff can be done with one of the options above. We do still see community – created device type handlers at the same rate as previously, so it’s not that people aren’t still coding. It’s just that they don’t need to code multiple variations of the basics anymore.
If you want to quickly search for existing custom smart apps, you can check the quick browse lists in the community – created wiki. That should show you most of what exists, and is kept pretty up-to-date. 
You can also see lots of examples of webcore rules (called Pistons) and get help with designing your own in the webcore forum, linked to from the webcore FAQ listed up above.
But as far as officially published specialty smartapps in the marketplace, I just don’t think we’re going to see very many more of them going forward. The whole design approach has shifted over the last two years.
But again, I’m just guessing.