Covering an entire runway in foam takes a lot of foam. There may not be any left for post-crash fire.
Foam reduces surface friction and brake action, friction isn't always bad, it makes it so that things stop. If you apply foam, you may overrun the runway because you are sliding.
Directional control is difficult on foam.
Evacuating people into foam means ...
In this youtube video showing landing gear maintenance operations on an ATR42, I'd say about 11 seconds for both extension and retraction.
A more precise timing could be gathered from an uninterrupted cockpit video, measuring from gear lever action action to "three green" on.