Writing in Spanish is a wonderful for me. I have no grammatical problems with it, and the rhetorical flow is much more satisfying. I tend to think of fewer options to express my ideas, so the process is faster. My personality is different, since I tend to think in the commonplaces of Spanish prose rather than those of English. Sentences tend to be longer and less choppy. I have to explain fewer things, because the Spanish audience can be presumed to understand things that might be unclear for English speakers. There is no need for translations into English, so that saves a step or two as well.
I would like to write only in Spanish, but I also like writing for readers who don't know the language, as in Apocryphal Lorca. Explaining Spanish literature to those who don't know the language, including my relatives, is also highly satisfying.