In a fast-paced (proactive) meta , pro players are forced to play safe to avoid letting the enemy team snowball out of control, which is boring to watch. Either that or one is steam-rolling the other for 25-30 mins, which also is boring to watch.
In a slow (reactive) meta (minus mage afk waveclear), pro players are forced to make higher risk plays to get a lead, and even retain it, in order to meet their win-conditions, which usually revolve around having priority for objectives to play around forced positioning.
Proactive plays being low-risk is boring because the one making the play almost always wins.
Reactive plays are always interesting because they are always high-risk, especially since both teams are usually relatively even in slower metas.