User blog comment:EmmiliaTheEmeralLeader/The Reason Roblox Removes Guests/@comment-26397550-20171114233030

The given reason was to benefit developers by removing a huge issue with a lot of main games: accounting for the existence of guests in the first place. Saving-based games were useless or messy, they broke some scripts, etc. They didn't like that people were needing to sometimes implement anti-guest scripts to not have their calculations broken and such.

Additionally, insisting signup increases the amount of players/signups for Roblox to offer for metrics to offer to advertisers and such. Since people can't play as guests, they have to sign up to play- that's an extra signup to their "Users Signing Up Every Day/Month/Year" stat, etc. Devs have some different game-based metrics, along with general income as guests can't buy anything with ROBUX while users can and stuff