Because he'd be a really rough matchup for Boom. Hyder has done a good job for them but I think Boom should be able to handle him judging from how he looked vs Tampa.
If I were the SF defensive coordinator, however, I'd put big 91 on the right side on top of Boom. Because Armstead's a powerhouse and that's Boom's weakness. But hopefully they are concerned enough about the Rams' run game that they don't risk that.
