This isn't even true. As well Fluffy, there's a reason Legatus is considered to have the best AS out of any commander despite that, and that's due to his AS animation.
If you look at actual win rates, Legatus sits higher on the win rates than a lot of commanders. These other snipers in general have to play high risk high reward. While legatus is low risk, moderate reward. If a vikka falls behind, she becomes invalid and useless. She will just instantly die without a way to deal with the situation. Mackwell has a little more survival, but will generally instantly die, and not contribute much if he falls behind due to his massive need to be in the lead. Itarri may be able to kite the best of these examples, but her short range dash doesn't actually do much vs commanders with MS augments and proficiency, a skrime during ult, or most gap closers. Xenos knock up is very good, but is has a very long cooldown. He has a giant window of weakness the moment he uses it. If all of these commanders, Xenos is the safest due to his Ult being so much powerful sustained dmg, and being the only high dps option where reflection won't kill the commanders, allowing him to continue to life steal.
Legatus has the longest range dash out of the bunch. Combined with displacement, and it is a nearly assured escape compared to the rest. While you may bash his W, its very easy for him to bypass all early game armor, meaning if he gets any lead, the armor shred will let him deal 100% of hsi AA dmg to enemy snipers. In short, the longer the trade goes, the more in his favor it becomes. Compared to Vikka who excels best in trades that are extremely short ((Due to the long CD on her Q, the trade has to be over 8 seconds, making it unfavorable to keep trying it that long without any damage abilities)). Itarri also excels at short trades, losing massive damage if her W is blocked by units and her misses. She has the worst sustained damage of all the snipers.
TL;DR - The longer the fighter, the better Legatus becomes.