As an administrator of a few small online forums, I've tried hard to minimize the burden of running the forums. Running even a single small online forum can take a considerable amount of time.
The forum software is best kept as standard as possible, aside from perhaps simple things like custom themes. Addons and modifications could go unmaintained, have security issues, or not be rewritten when the forum software deprecates features the addon/modification relies on. These addons become additional pieces to maintain. Keeping up with forum security updates alone can be irritating. The security updates rarely come at convenient times. I always have something else I'd rather be doing.
For example, recently someone requested that I install an addon so their Discord server could automatically be notified when someone posts. I looked at the addon, saw that it already was dropped by the original maintainer before being picked up by someone else. Installing it looked convoluted. I told the Discord administrator to use the built-in Atom feed with the right Discord bot, and recommended a particular bot. They now have the feed on their Discord server working. No changes to the forum were needed, which kept the burden on me to a minimum.
And when a forum is dead and basically an archive, the forum software can have issues (incompatibility with later standards, slowness, security problems if the software can not be updated). So making a long-term archive seems prudent. Some people seem to want the archive to be particularly fancy. I once had someone ask me to make an archive of a forum I'm an administrator on. They wanted it to be a "modern" React app with a powerful internal search feature, etc. As I recall, I said that I'm not going to develop that and prefer simple static HTML. No maintenance needed. Want to search? Try Google's site: operator. (In response to this request, I downloaded the entire forum with wget, which took a while and required some iteration to get the settings right as there were many duplicate and unnecessary pages saved. I haven't uploaded the archive yet as I need to fix some broken links and the forum isn't entirely dead yet.)
Given all of this, I can understand why someone would choose to make a subreddit over running a forum on their own server.
The forum software is best kept as standard as possible, aside from perhaps simple things like custom themes. Addons and modifications could go unmaintained, have security issues, or not be rewritten when the forum software deprecates features the addon/modification relies on. These addons become additional pieces to maintain. Keeping up with forum security updates alone can be irritating. The security updates rarely come at convenient times. I always have something else I'd rather be doing.
For example, recently someone requested that I install an addon so their Discord server could automatically be notified when someone posts. I looked at the addon, saw that it already was dropped by the original maintainer before being picked up by someone else. Installing it looked convoluted. I told the Discord administrator to use the built-in Atom feed with the right Discord bot, and recommended a particular bot. They now have the feed on their Discord server working. No changes to the forum were needed, which kept the burden on me to a minimum.
And when a forum is dead and basically an archive, the forum software can have issues (incompatibility with later standards, slowness, security problems if the software can not be updated). So making a long-term archive seems prudent. Some people seem to want the archive to be particularly fancy. I once had someone ask me to make an archive of a forum I'm an administrator on. They wanted it to be a "modern" React app with a powerful internal search feature, etc. As I recall, I said that I'm not going to develop that and prefer simple static HTML. No maintenance needed. Want to search? Try Google's site: operator. (In response to this request, I downloaded the entire forum with wget, which took a while and required some iteration to get the settings right as there were many duplicate and unnecessary pages saved. I haven't uploaded the archive yet as I need to fix some broken links and the forum isn't entirely dead yet.)
Given all of this, I can understand why someone would choose to make a subreddit over running a forum on their own server.