Not entirely sure that's true. Contador couldn't give a toss about a TdF podium having won it several times, and was attacking all over the shop. Just no longer had the legs or team to back it up. Quintana has 3 TdF podiums already, if he saw a chance to go for a win he's not going to play it safe to protect yet another podium, but again he just doesn't seem to have the climbing form that he did a couple of years ago. I think a 2014-15 era Quintana would have done a lot more damage to Dumoulin in the mountains and won this year's Giro.
I think it's more the case that the Tour is the one race where all the strongest contenders show up peaked and with their strongest team, and that strength in depth makes it very difficult to make attacks stick, which leads to what appears to be more conservative racing.
I think it's more the case that the Tour is the one race where all the strongest contenders show up peaked and with their strongest team, and that strength in depth makes it very difficult to make attacks stick, which leads to what appears to be more conservative racing.