The Atlas was the OG 100-ton 'Mech so it has a lot of love for it built up through the decades. Besides being the game's mascot for a while, the stock AS7-D Atlas is also a just plain solid 'Mech for 3025. Particularly for that age, having something you can chuck at enemies at medium to long range while closing in is useful, and even when you're stuck in point-blank combat (where the AC/20, SRMs and MLs all work perfectly together) you can always fling lurms at something else in the battlefield if you have the heat left over for it.
Also, the thing everybody loves to forget about 100-tonners is that they don't just carry their weapons. An AS7-D carries a combination of AC/20, SRM-6, LRM-20, 4x Medium Lasers (flipping them forward is trivial) and a point-blank, ammo-independent AC/20 (kick) or a pair of point-blank, ammo-independent AC/10s (punches). No matter how much ClanTech fucked with that paradigm, BattleTech is still a game of close-range skirmishing. For the Atlas in particular having all its heavy-hitting weaponry mounted in the torso (along with two MLs), means you can just trundle on to a target and go full rock 'em 'sock 'em robots while losing almost no firepower in the process. Which is something the King Crab or many other iconic Assaults can't do nearly as well.