To collect the infinity gems?
Infinity war movie take place in DCEU earth
Before he died Heimdall sent Hulk/Banner to the Batcave to warn Bruce Wayne and the rest of the League members about Thanos's arrival
In a desperate attempt, Bruce decided to revive Superman with the Motherbox
JL got 2 days preparation
Thanos already has 5 gems (Power, Space, Reality, Soul and Time) when he arrived in Gotham
He knew Vision is somewhere near the Gotham protected by the JL