Commentary Par XXIX 15

The Latin verb form subsisto is used here, as Bosco/Reggio point out (DDP Bosco.Par.XXIX.14-15), voluntarily (Dante had used the Italian form of the noun substantia [sussistenza] at [Par XIII 59] and easily could have used sussisto here, which rhymes perfectly with visto and acquisto).  And so we may conclude that he wanted the Scholastic flavor that the Latin term affords.  See [Par XXIX 12], where the parallelism with the Latin word ubi causes the reader to realize that a perfectly usual Italian word quando is there a Latin word.