It’s very, very simple.
Players will go as fast as they can survive. The less threatening the content, the faster they will go.
What level are Heroic now? 389? Something like that? Less?
You have 420s incentivized to run Heroics, because they can get crests for an upgrade and, especially at that level, Heroics are trivial content.
TW doesn’t scale very well, so, again, most players are OP, and the weekly reward offers them an upgrade. Even with lower level characters, the group can blaze through the content. Even with chain pulling I don’t think I’ve ever recently seen a healer cry out that they need mana in a TW.
You may think that M+ rewards speed, but, rather, it rewards power, coordination, and encounter knowledge. It rewards clock work precision group dynamics that manifest as speed. But speed is not the direct goal, rather its the only way to measure the group dynamic. If a group succeeds within a particular time frame, they could only have done that with appropriate power, pathing, and encounter execution.
M+ is scaled to punish groups that go too fast. As much as it rewards speed, it punishes failure. A couple of group wipes, and the run is broken. So it’s a razors edge. That’s one reason folks like it. As much as they want to burn through these instances, they like that they’re on the precipice of disaster, they like working like a well oiled machine and triumphing.
But lower level content does not scale, yet attracts high level players who can afford to be fast and sloppy, so they are.