I'm thinking tax purposes. When you're a small outfit, the extra paperwork burden that comes from hiring people in other countries, without visas, probably would make it not worth your while.
In the US, you have to get the applicant through a lottery system. It used to be a high percentage would get through, but the last I heard, it was more like 50/50. So, if you hire a lot of people, and have 30 with visas and get 15, that might be ok. But, if you only need 1, it's too high stakes.
I'm thinking tax purposes. When you're a small outfit, the extra paperwork burden that comes from hiring people in other countries, without visas, probably would make it not worth your while.