I think @Netheos has said elsewhere on this forum that it's a request from GW, so that's that.
[Edit] Not 100%, but probably for the reason he gives here: https://forums.focus-home.com/topic/2858/why-the-unnessasary-feature-ruining-and-frankly-kind-of-stupid-restriction-on-mixed-race-teams/2
Avec c'est changement je pense que l'équipe serait plus fun à jouer et elle serait aussi un peu plus forte. Quand pensez-vous ?
So you want to take what is already a strong team, and make it stronger?
Do I detect a hint of favouritism?
If you haven't already, download the LRB6 rulebook (might be called the Icepelt Rulebook in some places) - this has a lot of the fluff that GW removed for the CRP.
Example download link: http://www.mediafire.com/file/6j0488vhip661ao/LRB6.zip but there are many others.