It is usually due to “budget cuts” as the easiest way to kill a project is to defend it.

Juno Jupiter flyby

Maven mission to mars

New horizons kbo flyby

Terra mission-earth science satellite

Aqua mission -earth science satellite

DSCOVR

SLS-which may actually be a bad program but is a good example of the political issues with NASA vs senate.

NASA is too beholden to politics… You can’t do 7 year builds and missions when the Senate flips every 4 years and has to kill everything the other side did on principle that it has a D or R attached to it. Everything is political.