Why not Warhammer Fantasy Battles?

Not as famous, but hope to see such a crossover one day. Really want to see how the winds of magic map to the colors.

They're doing a Age of Sigmar Secret Lair; I'm not sure if that is the same as Warhammer Fantasy Battles but I believe that is part of the fantasy side of Warhammer.

I do think 40K is more famous and I think just allowing them to stretch more is a boon for them in a Universes Beyond product.

For the record here:

Warhammer Fantasy Battle (WHFB) was the original Warhammer. Warhammer 40,000 (WH40k) came after, porting the ideas to a space setting. It does seem like WH40k really took off as the version that resonated with people.

WHFB had an event called "The End Times", in which the entire world was destroyed. Age of Sigmar (AoS) is a successor to WHFB. Essentially, AoS takes place in a more esoteric fantasy setting that emerged from the ruin of WHFB's Setting -- in a lore sense, if WHFB was Forgotten Realms, the entire Forgotten Realms blew up and then AoS started as Planescape, technically the same meta-setting but different in a lot of other ways.

The transition from WHFB to AoS ended up porting in Space Marine lookalikes, eliminating, merging, or rebranding other factions, and shifting the gameplay dramatically towards a form more similar to 40k. It was... extremely controversial. A lot of WHFB fans felt betrayed -- a lot like Magic fans when the Mending first hit, but much more serious because, well, the entire setting did go entirely kaput. It would be like Wizards completely destroying the Multiverse and every plane in it, and then introducing Magic 2.0 with drastic rules changes and a new setting that shares some elements with the multiverse but really isn't the same.

In a sense, the angry WHFB fans have been vindicated over time not by AoS failing (as many hoped or insisted it would) but by interest in the Old World (WHFB's Setting) growing and reviving in a post-WHFB world. The Total War: Warhammer series, for instance, takes place in the WHFB universe and GW has announced and continues to be hard at work (including showing some of their work) on a WHFB revival called "Warhammer: The Old World" that also looks to bring back regimented armies with default square-bases, but remains a long time cooking.

So in a sense, the answer to the OP question "Why not Warhammer Fantasy Battles?" is that WHFB is currently dead. Any magic player knows that things in the Graveyard can always be recurred, but that's where it stands right now.