Originally, boards and other php or similar webapps not only had server storage limitations, but there were serious memory and processor limitations as well, not even considering things like internet connections through CSD or dial up. Sometimes boards were hosted on a desktop computer stuffed in somebody's closet.

Mods and admins normally would archive posts, the older IRC/usenet gen admins even chastised most users as being stupid for not understanding this while resurrecting old threads.

That spread to the Danny-do-gooders, you know those who aren't actually admins but cruise around looking for off topic posts they can ravage or those searching for posts with misspelled words that just need to be pointed out.

That carried over to the "internet forum culture" you speak of and why users were generally against it. But originally, there were very good reasons for limiting old posts. But it was always a dumb idea in terms of finding and access to information, especially for those willing to use the search button.

These days, cpu, ram, and for the most part storage space is really cheap. A decent VPS can be found fairly reasonably. The expense today is completely different now. The backend work by an actual sysadmin requires more time, effort and expense especially if you can't find someone to do the work on a volunteer basis. So now many site admins find it easier to just keep to the old way of limiting databases or limiting php rather than look to optimize the system because, well it's just less work and most ppl dont know any different.