I cannot stand for the author of that article (interesting and nice find - many thanks!), but 105 mm was considered both proven and with enough punch. In `70s USA just developed new generation of APFSDS ammunition (with DU monobloc penetrator), and had a hope of "new life" of 105 mm gun. Also, there were a ideas (that changed to plans in early `80s) of using 105 mm gun with longer barrel (L/65 instead of L/51). Since circa 1975 German 120 mm gun still was only experiment, and own US experience with smoothboore and APFSDS were not very encouraging to swap to 120 mm. Also, cannot forget about other reasons - national and technical pride, economical issues (need of new licence and ammunition production - and what to do with tons of 105 mm ammo?).
Note in race to arm XM1 there was also British competitor, with their 110 mm and later 120 mm rifled guns.