Most manufacturers have several months of inventory on lots, and factories on the same continent as the cars are sold. If inventory of a given colour starts running low in a given area, it's no problem to make more. Tesla, however, has to try to estimate how many of each possible config will be bought in what market, 1-2 months in advance. It's a much harder inventory problem. Reducing the number of colours is extremely useful to Tesla in this regard.
Once Tesla has a factory on each continent, then and only then might I expect more colours. Even then, though, the lack of dealership-lot inventory will always make it a harder challenge for Tesla than for others.