It is way faster to dump 17 .22 LR cartridges down the magazine tube from the muzzle end, than to insert them one-by-one through a side port. In fact, a buddy gave me an aluminum tube that might work as a speedloader, with a little hammer and file work.
I think the higher powered centerfire levers were "the" fighting rifles of that era. That being the case, it is easier and more ergonomic to top-up tube mags from side or butt ports in between shots, on the fly, or while keeping the rifle mounted, on target (almost), ready to fire and doing all this behind cover.