It is also a good idea for any business to have more than one product.
Generally, the interest in Fantasy seems to have strengthened in the last decade, sometimes even at the expense of science Fiction.
This trend seems to change in more recent years.
As a company.... if you only do Science Fiction books or games, such trends can put yourself into significant trouble when the majority of the market is in Fantasy "mood". And vice versa of course.
Being versatile significantly increases medium and long term success probabilities for any company.
I could also imagine, that the devs want to try out something new once in a while. I mean you are pretty trough with space ship designing and after all that work you might find designing magic creatures a welcome change.
