Angry Grimace
Two cannibals are eating a clown. One turns to the other and says "does something taste funny to you?"
Okay, so, answer me this: why is Jace in Shadows Over Innistrad?
Not as a card, from a narrative perspective, as an author's character. Why is Jace here? There's already a character with a personal stake in the events that's more interesting and thematically appropriate. There's already another character that's clearly capable of filling the role of protagonist-cipher/outsider investigator. There are other Gatewatch members you could certainly use, if you felt it was explicitly necessary to tie those threads together.
Is the answer "because Jace has ties with Lilianna and we want to do something with her"? Okay. Why does Jace have ties with Lilianna, when they already explicitly had her character tied to Garruk? What was the actual reason for linking Lilianna and Jace in the sort of slapshod, uninteresting way they are to begin with?
I don't want to belabor the point, but Jace has been overused in their meta-narrative for years, and he almost never adds anything over just using another character entirely. He's like a professional wrestling face that the company wants to push no matter how much the fans groan and refuse to cheer whenever he appears in the main event.
Because they wanted storyline continuity from block to block, which means one of the Gatewatch needed to go there. Gideon is the face-character of BFZ and Chandra is presumably the face character of Kaladesh. The only other option is to send Nissa to Innistrad, which would be really bizarre in my opinion.
Besides, Jace was mostly superfluous to the plot of SOI (I don't know what the plot of EDM is yet, obviously). He mostly was there just so there was a familiar character there to experience the plot.