Backward compatibility. It will refuse to die /forever/.

Also simplicity (for the printer vendor), codepages means you just take your input byte and index into an array, UTF8 means you need to implement UTF-8 decoding (which is simple but not free and now you need to deal with decoding errors) and automatic check/swap of your raster maps in the printer. This is all logic which codepages foist onto the driver developer.

Dealing with decoding errors is simple, you just substitute U+FFFD any time you hit one.