Richard,

It's easy.
As you just found out, there is world beyond linear sources. And, there is also a considerably wide range of applications, where you rely upon someone else power supply, over which you have little or no control. Of course, you always can design a DC/DC converter of your own of any qualities, but it's easier/cheaper to get a 7805 and a three-legger reset IC - only a few designs require more than this.

I'd bet it's just simply that more and more people put the microcontrollers in more and more places, that's all. So the environment changed. It has nothing to do at all with Intel, AMD^*, Siemens^*, Philips etc. The people at Intel are clever enough to realize this and would they be interested in the microcontroller business - which they are not any more, what a pity - they certainly would come up with something, simply working.

However, it appears that it is a specifically '51 problem of not having decent reset implemented, I'd guess it's an intertial problem. AVRs have reportedly good internal resets (which might come as a surprise as the AT89Sxxx's have crappy or none, and it's still the same company, but apparently two different groups not communicating to each other enough); so do the modern PICs (they have resets for ages, but the older were said to be crappy, too). I don't know for Moto/Freescales nor others, maybe Russell or somebody else might shed some light into this topic?

JW

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* your choice of "benchmark" processor clearly indicates how old they are - what was the last year AMD produced '51s? And Siemens? Also, the semiconductor business of Siemens is called Infineon for quite a couple of years...