2014 Subaru Impreza

I took my 2014 Subaru Impreza to have an oil
change at oil stop.  They told me I needed a
radiator flush. I didn’t have enough time to do it that
moment.  Three days later my check engine light
went on. I brought it to the Subaru dealership they
said the engine had been blown!  They asked me
for my maintenance records when I showed them
the oil stop receipt we noticed in the corner the
remarks read that the radiator coolant and water
were unbalanced, corrosion and broil over had been
compromised.  Contamination present  in the
system .  why didn’t the check engine light come on
sooner ?  Now the car is unsalvageable