If you're looking for forum software there's a good chance you've already encountered bbPress!
So why would you choose anything else? Well, we can think of three reasons you might consider - in addition to those outlined in our DETAILED REVIEW of Simple:Press vs bbPress.
This is far and away the best reason to use something other than bbPress.
bbPress and almost everything related to it is free. That should be a good thing, right?
Well, not really.
Even though bbPress is free and has over 200 free add-ons available for it, all of those were written by different authors.
This means that there is no one group responsible for making sure that they all work nicely together. And many of those add-ons have been abandoned.
Who can say if the one you adopt today will be around tomorrow? A commercial plugin like Simple:Press ensures all of its add-ons work together - and you have a place to turn to for support when they don't!
2. Approach To Forums
Every plugin has a different way of attacking a problem-space and bbPress is no different. It could be that you simply might not like the way it tackles forums therefore you might find out that you prefer a different plugin.
For example, bbPress screens are tightly integrated into the sometimes archaic WordPress UI and makes liberal use of Custom Post Types. Custom post types in many cases can also be a performance hog on larger sites.
So, an alternative plugin such as Simple:Press that primarily uses custom tables with custom indexes coupled with an alternative UI - well, that might be a much more attractive option for a WordPress forum plugin.
bbPress is huge. Or at least it can be. Other plugins might have a smaller code-base while still doing what you need it to do. This could mean that your site runs faster than if you implemented bbPress.
4. Bonus Reason - Popularity
bbPress has a 4 star rating on wordpress.org. Most users would think that this was quite a good rating. But, given that it has 288 reviews, its actually on the low side for something that has 300,000+ active installations.
A plugin that is this popular should have something much closer to a 4.5 rating. And a much higher number of reviews. That extra half a star might not seem to be much but it takes a good number of reviews to move the needle that much. (You can see other plugins with a similar number of installs on wordpress.org).
What does this mean? It generally means that users aren't happy enough to want to rate the plugin - hence the low review count. And a larger than normal percentage of those that do bother to review it are unhappy.
A full list of lesser known alternatives is included in this January 2019 wpbeginner.com article: 10 Best WordPress Forum Plugins (compared). Be warned - some of those listed there are actually WordPress themes and not plugins. Still, its worth reading so you can understand all the options out there!
And finally, don't forget to check out our head-to-head features review of Simple:Press against bbPress!