Because, as I said early, common sense doesn't sell. How do you monetize "pay off debt with an interest rate higher than your RoR and not debt lower than your RoR"? You have to take an extremist view to get people's attention. Hell, even MMM does this with his "complainypants" and "facepunch" and other bullshit surrounding generally good ideas.
If "Spend less than you make" and "don't stay in debt, and if you are, pay off the high interest stuff first"
worked, Dave Ramsey wouldn't be needed.
Even though it's "sub-optimal," his debt snowball is popular
because it works. It works for people who have tried and repeatedly failed to pay off debt using the logical method, because it's tapping into something different. The "Debt free!" calls are regular motivators for other people as well.
If you have your finances in order, Dave Ramesy is mostly useless, and can safely be ignored. But the reality is, he's helping a lot of people get out of debt. And his methods work where "common sense" hasn't. So, from that perspective, he's worth a good bit.
And regarding mortgage payoff/etc, there are other things to consider than just "on paper wealth." I fully intend to pay off my mortgage early (when we eventually have one), because that's something concrete, even if the economy tanks. I'd rather not have to suck out money from investments during a downturn just to pay my mortgage, and paying off a mortgage has much higher yields than a savings account (best I've seen is about 0.95%). *shrug* I'm aware it's not "optimal," and I intend to do it anyway, because I'd rather own my house/car/etc outright than possibly make a bit extra on investments.
... and I'm pessimistic about our post-peak-energy future, so I sit firmly in the "Past performance does not guarantee future results" camp about the stock market.