Allegro did have one or two MD-80's some time ago. My brother flew on one MEX-CUN-MEX. I will guess that they got the B727 for a cheaper price and they return the 80 to the leasing company.
Allegro needs a replacement for the old B727's and the MD-80 will be excellent since they burn less fuel and they only require two pilots instead of the three the B727 uses.
I was reading on the newspaper that allegro is not very good financially; they had to close their regional airline in Nuevo Leon and they had to stop the development of the schedule service division.