Ask HN: Why do some social apps don't allow periods or underscores in usernames?

Twitter doesn't allow periods in the username; same with Telegram. Gmail treats email addresses with periods and no periods as the same, but doesn't allow underscores. Instagram and TikTok allows a mix of periods and underscores in their usernames. Is it simply a matter of choice by the creators or there's some technological or design-focused reason behind why social apps don't allow either periods or underscores in usernames?