Unions tend to target a specific industry, and “work from home” encompasses a lot of different kinds of work, in a lot of different kinds of industries.
Work from home can range from anything from an IT worker who spends most of their day in SSH to a Customer Service Representative who spends all day on the phone with customers.
I don’t think there are necessarily any rules stating that a union has to be people in the same industry… that’s just generally how it’s done. It’s definitely likely possible, since people who work from home have similar needs in terms of union representation., however it might be confusing to have all those different types of work under one union representation. I don’t know, someone else probably knows more.
Organizing people who are by their very nature, separated from one another, seems like a tall order, but one worth exploring.