There isn't the margin in the price of an economy class ticket to provide accommodation on a stopover. The ticket already has to be priced below that of a direct flight to allow for the inconvenience and extra travel time.
Stopover packages work if the location is interesting enough to make the visit worthwhile even if it wouldn't be worth a special trip. The airlines will get a commission from the hotels and tour operators they use. A day or two in Dubai en-route to the far east or Australia makes travelling economy bearable, allows for easier time zone adjustment and gives an extra to the holiday
The margin on business class tickets is much larger and $150 for a hotel room still leaves a decent profit on a $4000 ticket. These passengers are less price sensitive than economy ones and need more incentive to take an indirect flight.
I would quite happily do 14 hours in business class but if I was in economy I would break it up into two legs with a night in a hotel in between.