if someone has played 0 games on the website they might have trouble connecting as some new users do, for subs this is bad for the player themself and the other players in the game.
the sub button at the top might appeal to new users who do not know how the site works as it is simple ("click here to play this class") compared to working out how the site works, these are not users i feel it is not smart to rely on as subs who will successfully join
generally people don't seem to read the lobby chat after the game has started, making it harder for any questions to be answered on how to connect. if a game needs a sub speed is important, i have seen people leave a game before it started causing it to fail due to simply lacking one player. during games being down one player is unfair for the team - being down one player for 3 minutes because a sub couldn't connect could be an entire round wasted on a koth map.
users in game already may have voted a player off for simply being bad (not sure if this is supposed to happen or not) and players new to the site may be new to competetive so there is a chance their first experience may be with some no-so-friendly players.
p.s. sorry about the duplicate report i submitted about the site not rendering properly i only remembered i'd done it when i checked "in progress"