Why isn't Xposed framework available on Google Play? So why isn't Xposed framework available on Play Store? Also, why are modules available there?
From XDA - Xposed FAQ,


  Why isn't the Xposed Installer available via Play Store?
  
  I [read: the developer] have several reasons for that. I wrote about it in this post. And as I learned, Google does indeed sometimes remove apps which interfere with other apps.
  
  From my point of view, you are free to publish the modules you develop on Play Store (at your own risk - Google might remove them). However, be careful if you plan to get money from them. Paid apps are stored in encrypted containers, which means Xposed can't load them at boot time.


From "this post", which explains a number of reasons,


  Currently, I don't plan to release the framework elsewhere. I had some vague plans to release it on Play Store, but there are some reasons for me not to do it:
  
  
  The rudeness in comments and ratings is unacceptable. This isn't for single apps, it's the overall feeling I have. I spend much time on Xposed and everything around, I don't want someone to insult me for it.
  Limited supporting capabilities. I think you can reply to comments now, but a dialog with the affected user would be required to fix bugs that may occur.
  Unexperienced users. In the best case, you just need to install the Xposed Installer, click the button, install a module and activate it. Restart and you're done. But problems do occur, sometimes resulting in bootloops. I hope that most users here will be able to make their phone boot again (if only by restoring a nandroid backup). I don't expect that from the average Android user, even if the description says "ROOT" and "BE CAREFUL!!!". I recommend these users stay off from risky modifications like this one.
  When somebody writes a module and publishes on the Play Store as well, their users might have trouble using it. Then these users will need to communicate with the developer of the module, but they might also contact me directly. Or if the developer doesn't know the answer, he might contact me. I would have to expect more support requests on different channels. More time spent on support => less time for private life and coding => less fun.
  Not sure what Google thinks about apps which interfere on such a deep level with their baby.