Real world - They're going to use a member of the main cast over some no name. As mentioned above.
In universe - Kirk knew Chekov better than any one currently on staff in engineering and wanted someone he actually knew in Engineering during a critical mission. There's no evidence the assignment would've been permanent if Scotty hadn't come back.
The strange thing is, this feeds into the idea mentioned in this old thread I posted months ago
that the stand-in chief engineer should have been a woman and not Chekov (of course, I don't agree with the person who suggested that.)