99% of phpBB installations were pwned and distributing malware. It’s a terrible model. If we want small distributed operation of software like that we need a revolution in software operability and quality.
The problem that remains is spam mitigation. Bots are very hard to detect, and there really aren't any good distributed approaches afaik.
I mean, why would you? It is quite a bit of technical hassle, even for an experienced admin. There's quite a bit of cognitive load just deciding what software stack to use, even before you run an installer. And then you have to maintain it too!
Compare that to less than 3 minutes of sign-up and creation of a new subreddit. It is an orders of magnitude difference in effort and knowledge required.
Security would be less an issue than back then, too. We learned a lot since early PHP and early Javascript's bad practices.