They had to play there because they were the last expansion team selected for 1969, a longshot to get a franchise. Expos beat out 6 other cities to get a team. Wasn't enough time to build a stadium, so they expanded Jarry Park instead.
If you were a National League fan from 1969 to 1973, one of the memorable things about Montreal Expos home games was this call. Claude Mouton, the distinctive public-address announcer at old Jarry Park, loved to roll the rs in names like Herrera and Hermoso. But he always took special delight in this catchers musical Italian name.