Generally because aircraft are extremely expensive to operate and it makes no commercial sense to run them if none of the routes available to your airline can be operated at a profit during a particular time period.
(There is a charter and short term lease market, if <i>someone else</i> has a route they can run that aircraft on over that period, but it&#x27;s probably not much of one for a 717 in Turkmenistan Airlines livery)<p>Aircraft also have periodic maintenance checks, some of which require the aircraft to be on the ground for an extended period of time. Same applies if the aircraft is having damage repaired. These may also show up as in temporary storage in Ascend&#x27;s database (source: I used to sell it)