Why can't blimps be economical for commuter / public transportation?
I'm guessing that it would absolutely be possible to make blimps economical. The question is whether it would be better than alternatives.
For public transportation, there are a number of common alternatives:

Buses. Low-to-medium throughput, relatively low speed, short distance, very high flexibility.
Trams. High throughput, relatively low speed, short distance, high flexibility.
Metros. Very high throughput, medium speed, medium distance, medium flexibility.
Planes. Medium throughput, high speed, long distance, medium-to-low flexibility.
Ferries. Medium throughput, relatively low speed, situational (i.e. water connections).

(There's also regional coaches and trains, which are the big brothers of buses and metros. Omitted mostly for brevity.)
The challenge for blimps would be to out-compete any of these other forms of transportation. It would be impossible for a blimp to cover the very high throughput offered by metros or trains, or the flexibility of buses. For larger distances, planes are much faster and are not nearly as affected by weather. They also have the benefit of trained personnel/drivers (mostly) readily available, more infrastructure, production and we know that they work. Plus you don't have to change people's habits, which is hard.
In short, there is nothing a blimp could offer that isn't already covered by better alternatives. Except maybe a nice view on a sunny day.