CPUs got faster, memory got larger, compilers got better.
Microkernels aren't a dead idea and they are even making their way into consumer electronics. (see Zircon).
In fact, even newer oses are backing into microkernels. Windows adopted similar microkernel concepts in Vista with the HAL. Android adopted a HAL with project treble in android P.
The steady march of modern OSes has been moving driver logic out of kernel space and into user space.