Why did narrowbody jetliners take so long to switch over to high-bypass engines?
Early high-bypass engines were expensive and unreliable.

The effort in developing new technology and designs, plus the cost of building a larger fan add cost. Then all of the new technology is inherently less reliable than existing proven designs.

The JT9D was notoriously unreliable, causing major problems in the development of the 747. Development problems and spiraling costs from the RB211 bankrupted Rolls-Royce, had huge failure rates, nearly causing an accident (Eastern Airlines Flight 855), and ultimately killing the L1011. CF6 fan disk failures caused National Airlines 27, sucking a passenger out the window, and the fatal United Sioux City crash. 

Even what we think of as the most reliable engine today, the CFM56, when new had a fan failure leading to a fatal crash (Kegsworth), combined with water ingestion issues that nearly lead to a crash (TACA 110).

When you have such compromised technology, you narrow it in the place that makes sense. High bypass engines brought high efficiency, reducing fuel burn. They also had more thrust, allowing heavier planes, carrying more fuel. Both advantages combine to enable long-haul direct flights which passengers like. These are flown with widebodies, so naturally these engines were used in widebodies. 

Short-haul narrowbodies carry less fuel per flight, and can refuel frequently, so there wasn't an advantage here. Only as the compromises with high-bypass engines were solved, then did the technology trickle down.

You see a similar thing today with carbon fiber composite fuselages. They're expensive to build, expensive to fix, and there is uncertainty over long-term costs. But the weight savings opens up ultra-long-haul flights, hence the A350 and 787.