I'm going to say it was because the early 'Burbs were built off of the existing panel truck platform. It wouldn't make sense to tool up a different stamping to add an extra door for a relatively low production vehicle. You gotta remember that at that time, anyone really needing to transport people frequently would have just got a regular passenger car based wagon, not a truck based one. I will admit, though, a side door would have come in handy even on the panel, so the cost could have been justified. Plus, if they'd done that, GM would have had an exclusive to the market.
