I don’t think we fully know the answer to this question though it is a very good one. In some tissues, even in the presence of oxygen, pyruvate doesn’t go to the mitochondria. For example, the eye is one of the most metabolically active tissues in the body. Oxygen is present but ~95% of pyruvate is converted to lactate and ~5% enters the mitochondria. Pyruvate transporters on mitochondria are present, just rarely used.

I think it’s highly dependent upon the concentration and regulation of lactate dehydrogenase (LDH) which converts pyruvate to lactate. If LDH activity is high, transformation of pyruvate to lactate happens more quickly than transport of pyruvate into the mitochondria. NAD:NADH ratio definitely plays a large role in regulating this.