I get that there are only 6 in paper, but still why?
https://magic.wizards.com/en/articles/archive/making-magic/odds-ends-zendikar-rising-2020-10-05
4th question down on Rosewater’s Zendikar Q&A
Basically because of the way the color pairs split during the set having 5 pathways would have left either an imbalance or an important pair not represented. So they decided to do 6 instead.
Thanks! @Mburn7
makes sense, even though it would have been reeeeeaaaally nice to have thefour last pairs too. can’t imagine why they wouldn’t , but oh wel…
OOOOOH we are getting the last four in Kalheim!!! yay!
Of course, WoTC is evil but not THAT evil 
No green/blue…
Sorry, but I think all 10 pairs should have been represented, both in paper and in MtGPQ… 
As said above, the other 4 will appear in Kaldheim, including the green-blue one (black-green, white-blue, black-red, green-blue).