If they can keep production levels the same as demand levels it gives them the ability to plan appropriately with a good few months buffer. If they ramped up and filled all orders then they would first have to start letting people go as they wouldn't be doing anything, and now they have no idea how to plan finances and accounting etc. By keeping the production rate at just the right levels they can have a constant rate/output/accounting. Steady work and output is better than high rate output and not enough demand.
You have to remember Tesla's business model is build on order, not build as many as you can and sell them to dealerships. There are no dealerships here. Tesla doesn't want to have a lot full of cars as they lose money each month the car is just sitting there. Hence the right model is to have the production line build only just enough to keep a few month buffer of orders. If all the sudden they get 5x more demand, yes then it would be necessary to add another shift or more lines as they also don't want customers waiting a year to get their car. If it wasn't for the fact that my one car died (murano, rod knock), and then my replacement (range rover Evoque) ended up being a lemon in October of 2012 right when tesla was starting to deliver, I probably would have bought a different car if I had to wait 12+ months to get one. I only had to play "musical cars" with my family for a few months but that wouldn't be sustainable and renting a car I thought was too much of a waste. It couldn't have been timed more perfectly for me.