That's because mediums seem pretty weak on paper in the hands of an unskilled pilot. But medium pilots love them. They're a good balance of firepower/speed/armor. They won't excel at any of those, but they can be very versatile mechs. It can hunt down lights (or chase them off your heavier mechs), it can move fast enough (pro-tip: ditch the default 64kph engines) to assist multiple fronts on the battlefield, it can brawl if required, and it can extremely effective close-range support (as a fast hit-and-run strike, or a flanker/skirmisher) for your heavier mechs.
Yes, a light can do some of those roles, but they won't have the same firepower or armor to deal or receive as much damage. Since lagshield has been improved, there have been a lot fewer lights for a reason. Once collisions are back in, reckless bad light players will be even more vulnerable in the harassing role.
A heavy (which seems to be the most popular at the moment) can pack a good punch and have better armor, and can still run at decent speeds. It can be devastating as a brawler or close-range support, but it won't maneuver or move as well as the medium. I used to also think that mediums might not be that useful. After all, I can put an AC20 and 2 SRM6 and a few medium lasers on a 'Phract, and still move at 64kph+speed tweak. Meanwhile, a default AC20 *OR* 2SRM6 hunch ALSO moves at only that speed. However, I can push the hunch to 80+ kph+speed tweak while still packing those weapons, and the role becomes very different. Suddenly that fast AC20 mech can run down lights if they get too close (and lagshield makes the shots actually hit now), and heavies/assaults will have a hard time tracking it down while it's flanking them. I can consistently deal more damage than Atlases on my team when I run a hunch. I also do similar amounts of damage as in my 'phract despite *on paper* having much less firepower.
Admittedly, I don't use mediums that much because I mostly use dragons (my fav mech in the game) for a similar role (they're really more heavy mediums than they are heavies). But, you rarely see dragons on the battlefield for similar reasons.