You're asking the wrong
You're asking the wrong question. You should be asking why 4Runner drivers are not switching to a Sequoia. If you want 7 seats and a 4x4 Toyota, your already driving a 4Runner. A Highlander is not an off-road vehicle.
I wouldn't go to the Sequoia because it's so expensive and gets pretty bad gas mileage. The highlander gets 21 the hybrid gets 35. That's a HUGE difference. Give me a large vehicle that's more affordable AND Hybrid or... even better a PHEV and we'll talk. But this would be the Grand Highlander that there is talk of releasing.
A Sequoia would be purely luxury item - OR for people who own show horses.