I can find discussions, without sources, claiming it is because they were afraid that the public would find it too easy to use, while others say it was easier to forge.
A little detail: it was forbidden just to Merchants to make use of Arabic numbers in book-keeping because they were easier to forge compared to Roman numerals or letters.
The ban came with the Statuto dell'Arte dell'Abaco del Cambio in 1299 ( source "L'eredità arabo-islamica nelle scienze e nelle arti del calcolo dell'Europa" by Nadia Ambrosetti.
The ban did not stop the publication of other books explaining the use of Arabic numbers (figure delli Indi / figures of the Hindi), suggesting that the ban was just enforced when it came to official book-keeping and Arabic numbers were actually tolerated in private use. Dante e la matematica by Bruno d'Amore
edit: corrected the name of the Statute