You could ask the same question about anything less than one’s best. Why not ask how anyone can daven in anything less than one’s finest?
I suppose you can state that since we have this audience with the King at a minimum of three times a day (and, in reality, as often as we want, since you can daven at almost any time), we have (for better or for worse) a familiarity with Him which allows us to approach Him in our less than best.
That being said, however, I would tend to agree that Crocs are probably inappropriate.
The Wolf