Not getting if yours was actually sarchasm, I'd say: because "reasons".
Just a small minority of people who play on console actually play on multiple console and/or PC, this because "reasons", the vast majority of people playing on console doesn't give a big "F#" of other echosystems beyond the one they actually play onto, you make friends on your game echosystem, you keep daily relationships on it at gaming level and beyond, the way is set now the trading system deprives every console player of his rights to trade with other users on the very same platform he has friends onto, IF he falls in the Cross Save TRAP.
The big problem is there are not enough nor clear messages and WARNINGS that, once you enable Cross Save, you'll be no more able to trade with users on your platform of origin unless they enable Cross Save as well.
From a PC user perspective, well, this thing just adds people to the trading pool, from a console user it just projects you in a world of foreign people with whom you are forced to trade, if you want to, while being no more able to do so with your friends onto your platform of origin, while it seems it can give access to more people to trade with, and in fact it is true, this system, as it is, is actually destroying the Warframe experience for console users, it can be a "non problem" for lone wolfs who doesn't have friends or strict in-game relations to other people, but is making every other console users felt in this trap a fish out of water.
All your friends are on "P" or "X" and happy to play the same game togheter, even helping each other trading something ? Once you enable Cross Save you are no more on "P" or "X" echosystem, but in an universal one that becomes practically a PC one, populated by people you don't know or play with, in the meanwhile all your friends keep to be happy playing and trading each other on your platform of origin in which you still believe to be, while is no more true.
I deeply believe that, until every Warframe account will be just a "Warframe account" regardless the platform, thing that should allow a full Cross Trading, there MUST be COLOSSAL WARNINGS on the fact that, enabling Cross Save to be able to access Cross Trading with PC users WILL CUT YOU OFF TRADING WITH USERS ON YOIUR PLATFORM OF ORIGIN.
I wonder if it could be possible for DE to just one day say "Well, startiing with this update a Warframe account is a Warframe account, no matter the platform on which is created", that would solve all problems.