I don't want to speculate about why there aren't enough dev kits.
But if MS want to really compete with PS5, I don't think obsessing over having the power advantage should be a big priority.
If any system has a power advantage this time around, I don't think it will be nearly as big a deal as it was with PS4 and XBO.
Because what made all those headlines wasn't just that PS4 was stronger. It was because the XBO versions often didn't reach industry standards of 1080p. That's why those articles and comparisons stood out. If both versions were 1080/60f, but the PS4 version looked slightly better, people wouldn't care as much.
And I don't think Scarlett or PS5 will have similar issues.