Because if you did, then you won't be eligible for any SPG Amex sign up bonuses for 24 months. If you've been following collusion in the news, then you know that amex and chase have colluded to limit sign up bonuses for people who have "recently" solicited bonuses from the other card vendor. If you haven't seen it, check out this link to see the details:
Aside from the major sign up bonuses, what is also included in these restrictions is those who have been long time card holders of the Marriott rewards card and were recently offered points bonuses to upgrade to the new marriott rewards premium card. Some took 10k points, some received as much as 50k, but either way that choice now proves to have been a bad one. I personally did not take the bait, but I know many have.