vacuum on table1 skips rows because of a query on table2

Currently I see the vacuum behavior for a table is that, even if a long
running query on a different table is executing in another read committed
transaction.
That vacuum in the 1st transaction skips the dead rows until the long
running query finishes.
Why that is the case, On same table long running query blocking vacuum we
can understand but why query on a different table block it.