There is a reason, and it is called php-bbdr.
About a year ago, I had been looking at parts of WordPress for another project (BridgeDD), and I noticed that the WordPress plugin system was modular. I wondered to myself "What would happen if I put it in phpBB 3.0?".
So I tried it. And it worked.
After I completed design work for BridgeDD 1.3/2.0, I turned my attention back to this amazing discovery. I tried it with all sorts of things, from phpBB 3.0 MODs to phpBB 3.1 development extensions to some of my hacks.
Everything worked, it was memory-efficient, and it was rock-solid stable.
The stability and memory efficiency made sense...the WordPress plugin API has been around in its current incarnation for almost as long as phpBB 3.0 has been around. It has been used by thousands of developers and millions of users.
That got me wondering. Was I some sort of genius for coming up with the idea to put the WordPress plugin API into phpBB? Maybe.

Then it hit me: why not fork phpBB 3.0.x and create a new version of phpBB with the WordPress plugin API? That was too big a project for me to do alone, so I showed this to two people, and one of them was very interested in the project.
php-bbdr was born.
After six months of development, we are ready to have people try out our creation and provide feedback for its development. Since this is an alpha stage, we are not making the product publicly available; anyone who is interested in trying out php-bbdr should either send me a private message here, use the contact form to send me a message (include an email address!), or feel free to reply to this post.
Here is the test site: https://php-bbdr.com/forum/index.php
I look forward to hearing from you!