1) I think the P4 may be headed towards a bit of a renaissance, simply because... alternatives are becoming unobtainium.
For Win98, it seems to me like one of those i865G/PE LGA775 motherboards that support C2Ds are a better alternative. Same compatibility as i865 with a P4 with a much easier to manage CPU (less heat output, etc) and much greater potential for an XP dual-boot. But at this point... the C2D-friendly i865 AGP boards are not exactly easy/cheap to find, so I think other i865 boards, potentially on 478, may come to be seen as passable alternatives.
I also wonder if people are going to look at P4s more for DOS, simply because again, slot 1/socket 370 stuff is getting much more rare, the more DOS-friendly thin clients are no longer plentiful, the good ISA sound cards have jumped in price too (I was looking at AWE64s on eBay... wow... now I understand why people selling homemade retro systems on eBay are using PCI sound cards) etc.
2) The P4 represented a time when Intel really took ownership of the PC platform and started to modernize it. Technically it started a little earlier, but the idea of the "legacy-free" PC was trendy. They were removing ISA slots, PS/2 ports, serial/parallel ports, even PATA. Probably some other more subtle stuff too that would affect DOS compatibility. Now, Taiwanese enthusiast boards resisted some of those changes compared to late P4-era big OEM systems... but...
Legacy-free is... not particularly appealing... for a retro system. You don't want the first generation to really support an incoming software ecosystem; you want the last (or greatest) generation to support an outgoing software ecosystem.
3) The other thing I would note is that nostalgia is an important driver of decisions. People want either what they/their friends/etc had in their youth, or what they read about in the magazines and web sites and wished they could have afforded at the time. Other than maybe a few Northwoods for a 6 month period or maybe one Pentium D 8xx that was a good overclocker (I forget which one), no P4 ever generated those strong positive emotions. The early P4s were outdone by Socket 462 Athlons; the later P4s faced Socket 754/939, all of which were much better received. P4s were what the Dells in the school library had, not something... sexy and desirable. And certainly Phil's Computer Lab on YouTube and others have done a lot of promoting 754 as a retro platform.
And I say this as someone who had a Willamette, a Preshot Deleron for a project, and family members with those ubiquitous Dell 2400/3000 C/Delerons. I'm the biggest Intel fanboy you'll ever meet. And yet... building a Win98 retro machine last year, I went with an AM2 AMD/VIA platform. Other than a few BIOS/chipset features that miiiiiiiiight have let me run 98SE on SATA, what advantage would a P4 system have had? I would have picked the P4 back in the day for perceived stability/compatibility/etc, but that isn't really a concern for a retro system - I'm not building it for multi-month uptimes running XP or Vista...