Why is a variable passed to the su command, but not an array from the same scope?

In the example below, why does $var get passed to the su in the start function, but not the array modules ?

#!/bin/bash
var=cat
modules=(
one
two
three
four
)

start() {
    su gleventh -c "for i in ${modules[@]}; do echo -- $i -- $var;  done"
}

$1

echo "out of function"
for i in ${modules[@]}; do echo -- $i -- $var;  done

The script above returns:

$ sudo ./test.sh start
-- -- cat
-- -- cat
-- -- cat
-- -- cat
out of function
-- one -- cat
-- two -- cat
-- three -- cat
-- four -- cat