There is a good reason. Long haul copilots become senior, getting trips with the best pay, Ultra long haul flights with multi-crew, so very, very little handling practice. Skills do get degraded. A move to shorthaul flying before a command course is very beneficial. I did it with move to turboprops- 6 flights a day was excellent for dusting off speed skills, even so I found a first command on 737 a challenge to begin with.
The way seniority works out, it seems to be a common progression that naturally falls into place. Our airline had many command course failures of pilots who were having to convert to a new type and upgrade their skills and speed of operation simultaneously. The trouble is, when life is just perfect, earning the best pay, going to the best places, on the best aeroplane, it is very difficult to voluntarily give it all up for a few years flogging around Europe in crap weather. But essential.
One of the drawbacks of staying longhaul for too long was possibly demonstrated in a United engine failure at SFO flown by a copilot with degraded handling skills- I recall he handled the OEI with aileron alone.