The reason why you'd want to use different account names that can't be linked is that it makes it harder to dox you. Consider this: what other online accounts have you linked to with your Twitter account, or share this specific user name? If it's an old name you've been using for years, someone with a bit of time and the power of autism on their side could well dig up stuff you probably don't want them to, or find other accounts you don't want linked to this user name.
Not recycling old usernames is an easy way to prevent all of this.