Why long routes by narrow bodies?
Airlines use narrow bodies for medium-long range routes mainly for 3 reasons.
- Market strategy
- Fleet structure
Some airlines choose to service medium-long range route with narrow bodies simply because they don't have enough customers for bigger planes---like the 2 flights you have mentioned, both of them are cancelled because of insufficient demand.
On the other hand, some airlines are likely to provide more frequent services even with smaller planes. Business passengers are the main sources of income on those flights, they will pay extra for better a time that better suits them. That is common in flights between main cities between the east coast and west coast of the US. American Airlines uses the 3-class A321 to service 10+ daily flights between JFK and LAX (2173 nmi).
In addition, some airlines provide business-class exclusive services on the selected routes. Due to its relatively small demand, the airline will choose narrow bodies even for medium long route. BA Clubworld between London city and JFK is such an example, they service the 3000 nmi route by A318 with 32 full flat bed seats.
Some airlines only operate narrow bodies, regardless of whether they are low cost carriers. So they service medium-long route only by narrow bodies, like Alaska 870 (Anchorage-Honolulu, 2418 nmi).
How long are routes by narrow bodies?
Both A320 and B737 series have "standard" 3000+ nmi maximum range when fully loaded, which can cover most of the "long" route. More accurately their MOTW starts reducing when range is over 2000 nmi. (Except B737-700ER)
It is hard to describe the payload/range in details. I would suggest you to have a look of it. (P.3-2-1 on A318, A319, A320, A321 & P.85-98 on B737 series)
ETOPS stands for Extended-range Twin-engine Operational Performance Standards, a rule which permits twin engine aircraft to fly routes which, at some point, are more than 60 minutes flying time away from the nearest airport suitable for emergency landing.
ETOPS is not a must when flying "long" routes, but it is necessary when flying over oceans or close to the Pole. Due to the limitation of the range, only a few routes require ETOPS, including a few transatlantic route (UK/Iceland - US East Coast), US West coast to Hawaii, as well as East Asia to Micronesia/Melanesia.