It’s a credit card terminal. How complicated could it possibly be or hard to get it right? Why is it even running on an OS in the first place instead of bare metal?
Because, if you're running on bare metal, you'll have to invent the OS.
Then where are you? You will now have an ad hoc, informally specified, bug-ridden semblance of an OS. Congratulations!
The bad guys will get a hold of a sample of your custom brainchild OS, reverse engineer it, and fashion a suitable attack. Then all they have to do is get on your network and deploy their code with a script, similar to the ones you use to update your system.
If the door's ajar, they'll get in!
Because they can. Because it's cool. To be fair, embedding something like Linux in an electric meter gives you access to protocol stacks and other platform software that work pretty well. But putting stuff like this under so much automation opens us up to remote attacks and we can't anticipate all of them. Was it really so bad having a 90 IQ guy drive around and read meters instead of sitting at home watching Jerry Springer?