Why is there no VO2 max on thredmill runnings

Because GPS is used to measure distance for outdoor runs, and GPS needs no calibration before use to deliver ‘accurate’ results. Distance is in turn used to determine pace, which from what I surmise is a key input into Firstbeat's VO[sub]2[/sub]max estimation method.

Was VO2 Max turned off for the Trail Running activity in response to complaints it messed up stats? Could be the case for Treadmill too? FP should be giving distance?

Was VO2 Max turned off for the Trail Running activity in response to complaints it messed up stats?

I don't know for sure, but I remember reading comments in this forum that it was the case.

FP should be giving distance?

The use of foot pods require proper calibration to give any semblance of accuracy in estimating distance, and I don't think there is a way for the watch to know whether a particular foot pod has been properly calibrated for/by the user. (I supposed you can argue that for failing to set maximum heart rate properly as well…)

Was VO2 Max turned off for the Trail Running activity in response to complaints it messed up stats? Could be the case for Treadmill too? FP should be giving distance?

Yes on Trail Run (at least on the 935 and F5) but VO2 Max on treadmills has never been enabled on any Forerunner device although not totally sure about the Fenix devices. The reason is largely as discussed - that a foot pod needs "calibration" for to get reasonable speed & distance and also treadmill running is generally "easier" than outdoors in any case. As such to save "complaints" that my VO2 Max indoors is "flattering" it was disabled.

I got a VO2 reading on June 7 (only) - treadmill and Fenix 6, HRM-Pro, Stryd only, score=46.  Why?  I run every day on treadmill with Stryd but only a reading on one day.  I've never run outside with GPS with my Fenix.  Yes, I am very fit at 64.