Sigh, the reason Juniper etc use BSD is because of the licensing, not because it's better at networking.
You can take BSD, modify everything and not have to share those modifications with anyone. So it's a great base to start building your own proprietary system. The real networking stuff in machines made by the vendors you name isn't done by the normal FreeBSD kernel but by software custom written by those manufacturers. Of course they will not back port this software to FreeBSD proper as it's the product they sell. If real heavy networking is going on its done by ASICs like Junipers Trio chipset.
If Microsoft doesn't have the intention to distribute their Linux version the licensing isn't a big deal. And there may be reasons why Linux is a better fit in their circumstance like the much more expansive hardware support.
Maybe you've heard of Arista ? Their kit runs on Linux. They must be stupid.