Because, save few exceptions (e.g. timers, early debug serial...) running drivers in supervisor mode isn't very smart.

Drivers tend to have high bug density, and supervisor mode means high potential for damage.

Much can be mitigated by running drivers in userspace[0][1], especially with some IOMMU help.

[0] https://www.cs.vu.nl/~herbertb/papers/minix3_dsn09.pdf

[1] https://wiki.minix3.org/doku.php?id=www:documentation:reliab...