Not in my backyard syndrome.

Because it doesn't exist yet.

Because you can save a quarter cent per unit by changing microcontroller vendors, but you have to use a new programming language for the new one?

Because you are involved in a lawsuit with the contractor that produced your previous version and using it would possibly be an admission it met requirements, even if your in-house developers had to completely rewrite it to make it safe?

Because new safety regulations mean you have to use a FIPS approved compiler and testing procedure?

Because it's a niche product and the only people in the world who know the problem domain work for you?

If you are designing a new car engine there isn't always of the shelf software already designed and tested. There are instances like the Ariane 5 maiden flight[1] where Ariane 4 software was reused.

[1] http://www.di.unito.it/~damiani/ariane5rep.html