Not only are they not using them, they’re paying out of their own pocket for SRMs.
This has been an issue for quite a while. Every single time it comes up, someone from Garmin or Slipstream will make some non sequitur PR statement that totally avoids the question - why aren’t you using your flagship sponsor’s product? They’ll neither confirm that they’ve given up on Vectors, nor will they say definitively state that they’re going to start using Vectors in the . The excuse du jour is that due to the Cannondale-Garmin merger, they couldn’t get the Vectors on the bikes in time for the Tour of California. Then the Giro rolls around and still no Vectors. It was the same thing last year - no Vectors during the Spring Classics, then I think Johan van Summeren actually had them for the TdF during one stage and then we never saw them again.
It’s like when your wife buys you something for your birthday that you think is awful (like a loud tie or a pink dress shirt). You don’t want to hurt her feelings but you really have no intention of wearing it. So you keep making excuses and hope she gets the message and never brings it up again.
So to answer your question - there’s never been a definitive reason from the team because the team has never actually stated that they’ve given up on using them (even though all of their actions speak otherwise). I read somewhere that the mechanics hate having to recalibrate the pedals every time they remove and install them. I’ve never actually used Vectors so I can’t comment on that from personal experience.