As the splitter rises in popularity, why do left-handers rarely throw the pitch?

The split-fingered fastball is the "it" pitch right now, but it is still rarely thrown by left-handed pitchers. Why is that?
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With Gausman and Yamamoto, the splitter is back in the spotlight for World Series Game 6

TORONTO(AP) — Mr. Splitty has returned. Showcased by World Series Game 6 starters Kevin Gausman and Yoshinobu Yamamoto, split-finger fastballs have been used for 6.8% of postseason pitches this year, ...