I can't answer that one, but my best guess would be to get them to the trouble spots quicker?

Perhaps the reasoning was not to add mobility. I think the mobility may have been a side-effect. The Polybian Reforms, to my knowledge, was the beginning of the empire-forming stages. They needed more available troops (hence the extension of the "who can be in the army" list) and manipular tactics were the order of the day. The hoplite phalanx of the Camillan triarii was very immobile and very expensive. Losing that lavish armor setup let up on a heavy expense, and losing the hoplite phalanx and the heavy armor allowed them to get to trouble spots faster and in better order.

That's my take. I'm open for correction by anyone who knows what they're talking about.