The number one reason is that BBEdit’s search and replace is better.

I can select text, make it the search pattern, then modify the text and make it the replace pattern. Then I can do a full or partial search and replace without ever opening the Find window.

If I want, I can open the Find window and edit either string. The search string will already be properly escaped as a regular expression.

vim’s search and replace is just a lot less convenient.

Beyond that, BBEdit is a Mac app through and through, so it feels very natural to use.