Because it makes more money.
And because 5e is futureproofed like unstable explosives.
The entire problem is the encounter assumptions. The designers never intended for other resource paradigms to work. So it's too late to fix it.
It might make more money, or they might expend resources and overall lose money - because the product doesn't sell.
Point is, it's a risk they don't have to take because not only is the fighter proven (by their metrics) it's the MOST proven.
Now, you could make the argument that since the fighter is so popular, any supplement involving it will also be popular and sell well. But it might also faceplant and actually take away from some fighter enjoyment. WoTC has proven pretty conservative and risk-averse on this front. Can't say it's not working for them.