Why some planes are scrapped way sooner than others?
In general, the retirement age of aircraft is around 25 years and most of the aircraft fleet in operation is well over 15 years old.



Source: airfinancejournal.com

There are various reasons for scrapping the aircrafts rather than selling them to other airlines (or other buyers, in general). 


Availability of Buyers- People should be ready to buy the aircraft on offer. In this case, the aircrafts were probably offered for sale at a time when tn the economy was in a bad shape and the airlines would've found it difficult to sell them.
Condition of the aircraft- Though the aircraft may be new, it has to be seen if it is properly maintained. In case the aircraft are not maintained properly or had the parts been cannibalized for spares, it would be difficult to find buyers as it takes a lot of money to put the aircraft back into operation.
Sum of Parts- In some cases, the sum of parts of the aircraft may be more valuable than the aircraft itself. Cost of operating an aircraft is quite high and the aircraft parts like engines etc are quite valuable. It would've made more economic sense to scrap the aircraft and sell the parts rather than (trying to) sell the aircraft itself.