"Pianist" is usually pronounced /ˈpiənɪst/, with /piˈænɪst/ as an acceptable variant only in the US and Canada, according to Wiktionary.
I'm not sure why the pronunciation /ˈpiənɪst/ would be more common though. If we were just adding an -ist, a suffix that as far as I know generally doesn't change the stress of a word, we would expect /piˈænoʊ/ → /piˈænɪst/.
Looking at the etymology, "piano" and is derived from Italian "pianoforte" (/pjanoˈfɔrte/). "Pianist" is derived from French "pianiste" (/ˈpja.nist/), which is stressed similarly to the Italian "pianoforte" – the /i~j/ glide is much more marginal than the /a/.