I recall reading somewhere that it was just a mistake by Bendis. Millar had plotted the first volume of UFF, intending it to be set before Ultimates, but Bendis's script threw in an Ultimates reference.
Spencer, Fialkov, and Humphries were all far from big names. Spencer had been hot for a second because of his Jimmy Olsen story, but Iron Man 2.0 hadn't done anything, and I think Ultimates was the first Big 2 work Humphries did. Fialkov would probably fall under the "rising star" banner, at least. As for Fiffe? ANU was the first thing he did with a print run of more than a few thousand issues.
None of that is a comment on the quality of those writers, mind you. Plenty of writers have great first stories, and a lot of big, seasoned writers write terrible books. But there's no way to compare the popularity of Fiffe and Spencer with Bendis and Millar, even as they were in 2000. There's a big difference between coming off runs on Powers or big work at DC and a popular self-published book or Jimmy Olsen backup story.
Also, you can't just mention the writers, because the popularity of the artists is important, and they were even smaller names. Even if Fialkov and Fiffe were rising stars, Amilcar Pinna and Andre Arujo sure weren't (again, not a comment on their talent). There's a big difference between Hickman/Ribic, who were undeniable big names at the time, and Humphries/Ross.
That said, I don't necessarily think it's a problem that Fiffe or Spencer or Humphries was chosen for those books. But there's no denying that Marvel never pushed the line post-2011 like they did before. Aside from the big event pushes, there wasn't much non-Miles talk from them regarding the UU. The Iron Man and Wolverine minis were launched with very little fanfare, for instance. The lack of push for Ultimate goes far beyond the creative teams. It's a pretty clear path back to about 2007 that Marvel started to put less and less emphasis on the UU as the event-driven nature of 616 took over.