Why does Spanish have so many diphthongs compared to other Romance languages?

Question

I have studied and known Spanish my whole life, and got a job at a University where I am allowed to take some free classes. Over the past three years, I have taken all the Italian classes offered, all the Arabic classes, and now I am currently enrolled in a Portuguese class.One thing that I am beginning to find really odd is how Spanish makes extensive use of diphthongs whereas these other two Romance languages do not. Is there a historical reason for this? Some examples off the top of my head are:> ES: No puedo ir mañana.> ES: Yo juego al béisbol.---> PT: Nãopossoiramanhã.> PT: Eu jogo beisebol.---> IT: Non posso andare domani.> IT: Io gioco a baseball.---With الديوان, Spanish uses a diphthong, but the diphthong [ua] is already almost there, just preceded by an [i].>ES: aduana>AR: ad-diwanBut in الطوب, there is just a change in the long [u] to a more informal [o]>ES: adobe>AR: aṭ-ṭuubOverall from Arabic, there doesn't appear to be any discernible reason from a Romance Language point of view as to what happens to the vowels, although if you can understand Egyptian Arabic, it looks pretty normal.----

Answer 1

Before answering, I would like to clarify that I am not a linguist, but I have studied the history of Spain extensively. The Iberian Peninsula was conquered and occupy by Moors between the years 711 and 1492 and during that time, Arabic became the dominant language of literature in the Iberian Peninsula and even became the preferred language for poetry and academia. As a result of this long period of influence, a large percent of Spanish vocabulary comes from Arabic (e.g. Ojalá and almohada). Italy did not have the same contact with the Arabic language which could explain the lack of dipthongs in modern day Italian. Additionally, Spain was invaded by Germanic tribes from the North before the Moors arrived and also has influences from Germanic languages like modern day German, which also uses dipthongs. In the case of Spanish as it's spoken in Latin America, there are frequently influences from indigenous languages (e.g. the 'tl' sound in Mexico that makes 'tlayuda').