Much of Europe was ruled by Arabs during the early middle ages. Granada had something like six centuries of Arab rule. Why is Malta the only place that still speaks a language with an Arabic base?
You cite Granada as an example of a former Arab territory that now speaks a non-Arab language (in this particular case, a Romance language). I think it is worth pointing out that Granada was conquered by the Catholic Monarchs, Isabella and Ferdinand during the Reconquista. The Monarchs later embarked on an anti-Arab and anti-non-Castilian campaign. Castilian was set as the national language, mosques and synagogues were closed, and Moors (Arabs) and Jews were forcibly expelled from the country. The expulsion of Arabic-speaking, well, Arabs obviously contributed to the decline of Arabic and Arabic influence on Castilian. However, Spanish does maintain some linguistic influence from Arabic - just not nearly as strong as its Romance heritage.
Furthermore, as more towards your question, Maltese is not a dialect of Arabic. It has a large influence from Italian and Sicilian, especially in the form of loanwords, and remains a distinct language from both Arabic and Italian. It’s also written with the Latin alphabet.