The reason is there probably isn't space. They probably wouldn't put a 17 gallon tank in there if a 50 gallon one would fit.I see no reason why they couldn't offer that as an option.
I mean, diff size fuel tanks are optional on other cars. The Porsche 911 allows you to spec it with an extended range fuel tank (24G vs 16G).
Usually retrofitted auxiliary gas tanks fit in the space vacated by an underbody mounted spare tire. The idea that they'd engineer a giant empty space on the underbody on a vehicle that was never meant to have a tire there is not believable.
For those that are new, the picture referenced was noticed by many on launch day. The guy in the video didn't discover anything new.