Why Cantarella?
Possibly because it is a pleasing, mellifluous word that will help a sentence hang together smoothly; possibly because the notorious aphrodisiac properties of that drug suggested it to Giovio as just the poison to be kept handy by folk addicted to the pursuits which he and others attribute to the Borgias.

Can you surmise any better reason?
For observe that Giovio describes the Cantarella for you--a blunder of his which gives the lie to his statement.

"A white powder of a faint and not unpleasing savour," says he; and that, as you know, is nothing like cantharides, which is green, intensely acrid, and burning.
Yet who cares for such discrepancies?
Who will ever question anything that is uttered against a Borgia?
"Cantarella--a white powder of a faint and not unpleasing savour," answers excellently the steady purpose of supporting a defamation and pandering to the tastes of those who like sensations in their reading--and so, from pen to pen, from book to book it leaps, as unchallenged as it is impossible.
Whilst Cesare's troops were engaged in laying siege to Ceri, and, by engines contrived by Leonardo da Vinci, pressing the defenders so sorely that at the end of a month's resistance they surrendered with safe-conduct, the inimical and ever-jealous Venetians in the north were stirring up what trouble they could.

Chafing under the restraint of France, they but sought a pretext that should justify them in the eyes of Louis for making war upon Cesare, and when presently envoys came to lay before the Pope the grievance of the Republic at the pillage by Borgian soldiery of the Venetian traders in Sinigaglia, Cesare had no delusions concerning their disposition towards himself.
Growing uneasy lest they should make this a reason for assailing his frontiers, he sent orders north recommending vigilance and instructing his officers to deal severely with all enemies of his State, whilst he proceeded to complete the provisions for the government of the Romagna.
To replace the Governor-General he appointed four seneschals: Cristoforo della Torre for Forli, Faenza and Imola; Hieronimo Bonadies for Cesena, Rimini, and Pesaro; Andrea Cossa for Fano, Sinigaglia, Fossombrone, and Pergola; and Pedro Ramires for the duchy of Urbino.

This last was to find a deal of work for his hands; for Urbino was not yet submissive, Majolo and S.Leo still holding for Guidobaldo.
Ramires began by reducing Majolo, and then proceeded to lay siege to S.Leo.But the Castellan--one Lattanzio--encouraged by the assurances given him that the Venetians would render Guidobaldo assistance to reconquer his dominions, resisted stubbornly, and was not brought to surrender until the end of June, after having held the castle for six months.
If Venice was jealous and hostile in the north, Florence was scarcely less so in mid-Italy--though perhaps with rather more justification, for Cesare's growing power and boundless ambition kept the latter Republic in perpetual fear of being absorbed into his dominions--into that kingdom which it was his ultimate aim to found.