Why do narrowbodies have longer life than widebodies?
The author does a very good job explaining the jet market. The key take-away is that it's all about economics. In many ways, narrowbodies may "wear out" faster as they typically fly shorter routes and accumulate more cycles in a shorter period of time. Pressurization cycles are a big factor in structural fatigue.  

But that's not the primary reason for retirements. Airplanes get retired when operating them is no longer profitable. Profit is generically income minus  costs. Income for aircraft is maximized by flying as much as possible (high utilization) with the highest load factors possible. Costs are primarily driven by capital depreciation, fuel, crew, and maintenance.  

The shorter lifespan he identified for widebodies is driven by a few key factors:  


Widebodies are a smaller percentage of the total fleet, so that even
a few "early" retirements will cause a larger shift in the average.
Widebodies are suited primarily for long-haul, high density routes.
So a shift in travel patterns can quickly leave a route
unprofitable. Covering that reduced demand will often mean shifting
to a smaller aircraft to keep the load factors up. Narrowbodies have
more inherent flexibility.
And what I would say is one of the most significant factors is the
shift of widebodies from 4 engines to 2 engines over the last 20-25
years. It's not a fast process as widebody construction is not very
high rate, but the cost benefit of 2 engine widebodies (B777, A330,
A350) means a lot of 4 engine aircraft (B747, A340) have been
retired in the last 10-15 years.