The Marians invaded two worlds. Other nations like the FWL, Lyrans, and FedSuns have suffered much larger losses within the last decade and no one is saying that they can't put together a supply convoy.
All that is needed is a single Invader-class jumpship with three Mule dropships.
The thing is, the Inner Sphere usually combat way closer to supply lines. From Gambilion to Ballalaba (Task Force Alpha objective) and Islington (Task Force Beta objective) you have at least 5 jumps. 2 mech regiments and accompanying forces for each planet, with the final destination beign Alphard (another 3 jumps from the closer planet, beign Islington). So their final destination is 8 jumps from the closest friendly planet /2 months trip each way). Even if they can ransack and pillage the marian planets along their way, just normal repair and maintenance, feeding the troops, ammo demands and assorted personal issues will degrade the combat readiness of both task forces, and the long supply line make them very vulnerable to Marian irregulars, or even normal pirates in the zone.
Even that single Invader with 3 Mules will be in greta problems to keep supplied both forces, and will be very vurnerable to raiding. And even if they have more JS and DS, as ARR said, the MoC, even with their long asociation with the CC/Andurien, still have a medium size fleet at best, and will be hard pressed to keep a reasonable number of ships on that duty while keeping with the supply of the rest of the MAF.