The Invader can carry 3 Overlord dropships, or a full regiment of Battlemechs. If you presume that in the Star League era, regiments may have been assigned their own transport capacity, then it makes sense to build a lot of Invaders. Regiments may not always travel together. Even if it's less efficient overall, it's more useful to be able to send one regiment over here and another over there without having a big jumpship make multiple trips. Just give them all their own ship and problem solved. It makes logistical planning easier.
Also in the Star League era, attacking jumpships was still common. A Star Lord or a Monolith makes a pretty tempting target. With Invaders, an enemy warship has to pick and choose which one to attack. You can jump in at multiple points and it makes it much harder for an enemy to respond. Let's say you had a fully charged corvette warship floating around at a lagrange point. It detects hostile jumpship arrivals and it can jump to their location and blow them up (along with any recently deployed dropships they're carrying). But with multiple jumpships, you need multiple warships to defend the system.
For freight purposes, a lot of smaller worlds just may not need a Starlord. Speed of transport may also be of the essence sometimes. One Star Lord lets you transport 6 dropships 30 light years in one jump's time. But 2 Invaders not only let you do that, they also let you transport 3 dropships 60 light years in one jump's time. That's a lot more useful when moving perishable supplies from a garden world to a place that really needs food that isn't right next door.