Re: A question from a young'un of 31...
The immediate reason at the time was simple... Money.
Windows 95, shipping in August 1995, had a minimum requirement of 4Mb of RAM, and recommended 8Mb of RAM.
Which everybody regarded as a joke. Sure, Windows 95 ran in 4Mb. And you could even run Notepad or Clock, too! But if you ran both of them together, your hard disk began to glow red hot as the swapping kicked in.
Realistically, everyone recommended a minimum of 8Mb and you should really want 12Mb or 16Mb.
For Windows NT? Version 2.51 was released in May 1995, and the minimum requirement was 12Mb, with the recommended 16Mb. And again, everyone laughed at that - you wanted 16Mb, preferably 24Mb.
Why is this important? Well, back then in 1995, RAM was for sale about 33 US dollars per Mb. (I couldn't find a reliable GBP price, so we'll have to use USD - sorry!)
That's before any taxes, too.
Let that sink in. If you'd bough a 486SX machine in late 1994, and it had 4Mb of RAM - fine for Windows 3.11! - then you were probably going to have to spend another 132 bucks just on RAM before you could spend your 90 bucks on the Windows 95 upgrade... Is your motherboard full already with four 1Mb sticks? Not unusual. You now get to throw away the old 4Mb, and spend ~270 bucks on four 2Mb sticks.
Eager to upgrade to Windows NT 3.51? Well, just double the prices... And start weeping, presumably.
Over the following few years, demand for RAM drove the prices down fairly quickly. But still, the requirements of Windows NT were a little more than Windows 95, and it never quite lost that reputation until Windows XP came out. (And perhaps not even then!)
There are other hardware issues, too. Windows 95 brought us Plug & Play, but that didn't come to Windows NT until Windows 2000 shipped in late 1999. And Windows NT also had a different driver model. Most manufacturers targeted Windows 9x for driver development as it had a wider installed base, so you were both more limited in the hardware Windows NT could run AND you sometimes had to fiddle with IRQs/memory addresses manually to get things working.
(Although in its favour, high end hardware like SCSI cards usually had much better Windows NT support and took a lot of the hassle out of hardware configuration anyway...)
Now on top of these costs and hardware issues, add on all the software compatibility issues that others have raised.
Windows NT was superb. Brilliant. Faced with a choice between Windows 98 and Windows NT 4.0, I jumped at Windows NT 4.0. But I was savvy enough to know how to select/handle my hardware, and how to tweak software to run under it.
It rewarded me with greater stability and reliability. But I wouldn't have recommended it to the average user on the street until WIndows 2000 SP6.
On its release, I didn't like Windows XP's crayola-inspired interface tweaks, or paying for what was effectively Windows 2000 Service Pack 7. But with hindsight, Windows XP was the version of Windows we'd finally wanted - a great blend of the plug & play and software compatibility of Windows 9x, and all the stability of Windows NT/2000.
And by then, RAM was so cheap (by comparison) that it really wasn't an issue. :-)