Martin Luther's "Bondage of the Will" gives a wholehearted defense of predestination, and as far as I'm aware, the two men considered themselves to have the same position on this issue as each other. Why did this particular belief come to define "Calvinism" when the chief reformer Luther seems to have already articulated this position?
There's an excellent answer to almost exactly this question in Matthew Block's article Why Lutheran Predestination isn't Calvinist Predestination (for First Things). Essentially, the later Luther and the following authoritative pronouncements in the Book of Concord (the common Lutheran doctrinal standard) have rejected the doctrine of double predestination as taught in Calvinist/Reformed churches (and enshrined in multiple Reformed confessional standards). With Lutherans explicitly distancing themselves from the strongly asserted double predestination, the similarities between Lutheran and Reformed doctrines of election took a back seat to the differences in both confessional self-presentiation, the framing of doctrine, and public perception of theology.
Fun fact: Martin Luther King Jr. wrote a very good term paper which goes into how the doctrine of election is situated in the particular theological contexts of Luther and Calvin.