Probably because Flash isn't as relevant as it used to be. Years ago poor Flash support was a dealbreaker for a lot of people that might have switched to Linux, which provided a strong raison d'être for open source Flash replacements. Nowadays, due to popular platforms like iOS with no Flash support, few sites rely on Flash for core functionality (though you still have cases like video quality being better for Youtube on Firefox with Flash because of the licensing issues with H264, most regular users won't notice this). "Millennials" (or whatever the fuck people in their 20s now are supposed to be called) that want to keep Homestar Runner alive are probably the biggest group that cares about having an open source Flash runtime today.

I still like Flash. It helped a lot of kids (myself included) get into computers, its authorship tools allowed non-programmers to create a lot of really cool animations and interactive things over the years, and technically it's still ahead of HTML5 in a lot of ways (the gap is slowly shrinking due to Adobe's stupefying negligence and apathy for the platform, but I still find that most websites with even the slightest hint of superfluous animation lurch and stutter embarrassingly today on my fairly decked-out PC compared to full-blown Flash animations on my Pentium). If Adobe hadn't bought Macromedia, and if their management had had the good sense to release the source to the Flash player client, I think the web would be in a much better place right now.