I noticed that none of the monsters from the Guildmaster's Guide to Ravnica have associated environments (with the exception of the Goblin Boss).

While I understand that Ravnica is a largely urban setting, it would be extremely helpful to have environments for these monsters since they can surely be re-skinned and used in other settings. Is there a reason why monsters from this book do not have environments?

It's probably because the GGtR book doesn't mention their natural environments, and "environment" is not part of the statblock. The monsters that have environment listed is because somebody went through the book, read the description and added the tags. In GGtR the details rarely, if ever, mention any natural environment - thus, they cannot add the relevant tag.

I believe the environments are taken from the random encounter tables they appear on in the monster manual and xanathar's guide to everything. So if a monster appears on say the forest table, they get the forest environment tag added.

So short answer: the environment tags are based on official information in the books. None were provided for GGtR.

Thanks everyone!