After IBM came out with the PS/2 series, many PC makers started using intergrated motherboards and proprietary designs that are not as upgradeable as the customized AT/ATX designed systems of today and yesterday.
That wasn't just after PS/2; Compaq long had a reputation for weird non-standard stuff. Shoot, for that matter the Tandy 1000 was a marvel of proprietory design that long predated the PS/2.