Actually, the reason is very simple. Without PM on Itch.io. Members cannot sent PM(Personal Mails or Private Mails) to other members on the Itch website. In fact, very, very inconvenient. Why not add this function to Itch? Is it difficult to add this function to Itch?
Viewing post in Account reset email to support
We feel that there are many other tools to facilitate private communication, and we don’t want to enter that space. There are many moderation, security, and safety concerns that arise when you open up accounts for private communication. We recommend that if developers wish to receive private messages about their work, then they provide relevant links or email address directly on their page.