I believe the launch windows are fairly constrained due to the precise positioning requirements as well as the requirement to avoid eclipses throughout the lifetime of the telescope. This study [1] was for a planned 2018 launch, but I'm sure there were similar constraints in this time period too.

In addition, the fuel required to position the spacecraft varies a lot depending on the day - and any fuel saved can be used to prolong the operational lifetime of the telescope.

[1] https://issfd.org/ISSFD_2014/ISSFD24_Paper_S2-6_Yu.pdf