The MG Manual states a reason for the more invasive approach:

Surgical approaches will differ among surgeons. For many, the preferred surgical approach is transthoracic; the sternum is split, which allows wide exploration of the anterior mediastinum. Transcervical and endoscopic approaches have less postoperative morbidity but do not allow sufficient exposure for total thymic removal and are not recommended when there is a thymoma.

Myasthenia Gravis: A Manual for the Health Care Provider (PDF) - page 18

It looks like if the surgeon needs to ensure through removal, the more invasive approach is indicated.

-Mark-