This is something I've been thinking about recently; I don't know if you'd agree.
Essentially, what I think is the case is that when people are deciding between quadras, they find it far easier to discern between Ne/Si and Se/Ni than they do Te/Fi Ti/Fe. Notice how Deltas and Alphas are confusing themselves with each other, and how there are plenty of Betas and Gammas who think they're each other. Ok, you get the odd Fi base type who practically knows they're Fi base, but not whether they value Se/Ni or Ne/Si, but it's far more likely that people will be struggling with the Te/Fi Fe/Ti question than the former.
Anyone want to take a stab at explaining this phenomenon?