There is more than one reason. What did the popup say?
I didnt take a screenshot of the popup. However I did google "Amex popup" and believe this is the message I got - "Based on your history with American Express welcome offers, introductory APR offers, or the number of cards you have opened and closed, you are not eligible to receive this welcome offer"
FWIW, this is the only Amex card I have at the moment and it always has some charges on it, typically in the $300-500 per month. Anyways I think I'll start spending a bit more on this card, especially the charges that are typically non bonus earning and then check again in a few months.