VG has several game design issues that keep captains in a bad place.
-Health ratios on everything: I believe that this is the primary issue holding back captain design. It turns even fragile, caster heroes into unwitting tanks. Casters should only be a shade more durable than ranged carries and mages. SEMC tried rectifying this with Lance, Lorelai, and Lyra when they gave them CP ratios, but there weren’t any CP items that provided any worthwhile utility at the time, which simply led to the ratios being ignored (Lance) or turning the hero into an absolute menace (Lorelai, Lyra).
-Health as the primary stat for defensive items: This goes back to my point about casters being tanks. Casters should be back line supports, and should be as or only slightly more difficult to take down than ranged carries and mages.
-Item diversity: The lack of hybrid items and the habit of keeping most defensive effects as actives basically turns captains into active item repositories. Their abilities tend not to be as impactful as activations of Fountain of Renewal or Crucible.
As it stands, the only heroes that can do reasonably well are those who provide team utility from afar. Catherine and Fortress are not popular captain picks because they have to position themselves in harm’s way to provide any benefit to the team. This is fine in teamfights, but weak ranged mages find it difficult to take advantage of.