I am guessing, but the DEC program is a money and asset deal. To hire a DEC takes only one training event and nobody being removed from the line. To upgrade a deserving first officer removes him from the line, requires a training event and another training event for his replacement.
That's the only reason I can see for the DEC program with the number of qualed right seaters available here already. Now if everybody in the right seat had limited time and experience, like many of the asian carriers, it might be justified. Go figure.......