Morimoto talks about Regigigas's ability like this:
Slow Start forces the player to wait 5 turns.
Once the Slow Start ends after 5 turns, though, Regigigas is actually much faster than it appears!
The developers basically wanted you to play Regigigas tactically: defensively for 5 turns, then offensively for the rest of the battle. In competitive, however, this makes Regigigas absolute trash. Imo, if they had a better Pokemon for it that could play offense-defense, it would be a really cool Ability to use; but, for now, all we have is Regigigas that sucks with Slow Start. :P
Upon further search, I found no reason that Regigigas should be hindered by slow start unless you count being massive (but there are bigger Pokemon that are faster than Regigigas). The dev's may have given the ability to hinder it because "it is so powerful," but it really wouldn't be outside of ubers/incredibly OP without Slow Start.
TL;DR: Regigigas was either given the ability to force a player to be tactical, to prevent it from being "OP", or because the developers give terrible abilities sometimes.
Hope I Helped!