Modern forums (WoltLab Suite, Xenforo, and Invision Community Suite are the big three), all look and function for a modern web environment, which includes mobile devices. Like it or not, most web traffic these days is on mobile devices.
One forum I admin has dozens of users signing up every day. One of the reasons I consistently see for why they've signed up is because we use modern forum software (Xenforo), that looks and reads great on their smartphones and tablets in comparison to other related forums that still use old forum software meant for desktop computers. Meanwhile, those other forums that still use old forum software meant for desktop computers are consistently seeing less and less traffic.
It's just the reality of the web in 2019. Instead of asking the masses to adapt to us, we've had to adapt to the masses.