Perhaps it's an artifact of selective memory or something, but it seems to me that 90Mhz Pentiums were actually pretty common. At its introduction it was expensive and rare for home use but it stuck around as a "budget option" for quite a while after the 133-200Mhz models came out. (The last Pentium desktop I let into the house, around 2002 or so, had a P-90 in it.) What *is* rare is Pentium 75-100Mhz machines equipped with the CPU-introduction-vintage "Neptune" chipset. Those "budget era" P-90s I kept tripping over all had either 430FX/VX boards or horrid PCChips/OPTi knockoffs, demonstrating that they'd been equipped with an "obsolete" CPU from day one.
It is *specifically* 90Mhz machines I recall as common. I think the reason was that near the end of the sales life of the CPU vendors would price the 90Mhz chips at practically that same price as the 75Mhz ones while still asking a "significant" premium for the 100Mhz models. (It'd literally be something like $75 for the 75, $79 for 90, and $99 for the 100mhz. For the seriously bottom-of-the-barrel crowd that spread would make the 90Mhz the obvious choice. I was in that crowd when I bought my first Pentium-class machine, and as a result ended up with... the AMD K5-90. Same price structure as the Intel chips but yet another $15 or so cheaper. Whee.)