What makes RPI so unique and why should there be a separate classification is a relatively easy challenge. Firstly Prog is by its sheer definition (and as I espoused in my own 1974 philosophy final essay, that I still am proud to have), a maelstrom of various influences mostly involving historical traditions in music, whereby classical, folk, jazz, electronic, ethnic, world and many other genres can fusion together with the typical rock format (guitar, bass and drums) and hence, evolve into something completely different and original. Obviously, groups like Ekseption started the whole phenomena by reformatting outright classical compositions from Bach, Beethoven, Mozart, Albinoni, Khatchaturian, Moussorgsky etc…adding electronic keyboards and brass (sax, trumpet, flute) while providing a strong rock beat (powerful Rickenbacker bass and drums). 

So the Italians latched on to this something fierce as the post-Woodstock explosion of sound and technology (synths) made them more in tune with Genesis, Pink Floyd, Deep Purple, King Crimson, VdGG , Yes and Jethro Tull than any other rockier offerings. It was a time when there were little or no boundaries as teens were avidly exploring new sonic landscapes. Popular music in Italy has always leaned heavily on the traditional songs of Italian culture, where vocals and lyrics have always been richly followed (opera) but also the canto populare , canzioni italiani etc..

“The music of Italy ranges across a broad spectrum of http://en.wikipedia.org/wiki/Opera - opera and instrumental http://en.wikipedia.org/wiki/Classical_music - classical music , the traditional styles of the country's different regions, and a body of popular music drawn from both native and imported sources. http://en.wikipedia.org/wiki/Music - Music has traditionally been one of the cultural markers of Italian national and ethnic identity and holds an important position in society and in http://en.wikipedia.org/wiki/Italian_politics - politics . Italian innovation in http://en.wikipedia.org/wiki/Scale_%28music%29 - musical scales , http://en.wikipedia.org/wiki/Harmony - harmony , http://en.wikipedia.org/wiki/Musical_notation - notation , and http://en.wikipedia.org/wiki/Musical_theatre - theatre enabled the development of http://en.wikipedia.org/wiki/Opera - opera in the late 16th century, and much of modern http://en.wikipedia.org/wiki/European_classical_music - European classical music , such as the http://en.wikipedia.org/wiki/Symphony - symphony and http://en.wikipedia.org/wiki/Concerto - concerto . Instrumental and vocal classical music is an iconic part of http://en.wikipedia.org/wiki/Italian_identity - Italian identity , spanning experimental art music and international fusions to symphonic music and opera. Opera is integral to Italian musical culture, and has become a major segment of http://en.wikipedia.org/wiki/Popular_music - popular music . The Neapolitan song, http://en.wikipedia.org/wiki/Canzone_Napoletana - - cantautori http://en.wikipedia.org/wiki/Singer-songwriter - singer-songwriter traditions are also popular domestic styles that form an important part of the Italian music industry, alongside imported genres like http://en.wikipedia.org/wiki/Jazz - jazz , http://en.wikipedia.org/wiki/Rock_%28music%29 - rock and http://en.wikipedia.org/wiki/Hip_hop_music - hip hop . Italian folk music is an important part of the country's musical heritage, and spans a diverse array of regional styles, instruments and dances.” Source Wikipedia

“The Italian progressive rock scene was born in the early 70s, mostly inspired by the http://en.wikipedia.org/wiki/Progressive_rock - - Britain , but with certain features of its own that makes some sources mention it as a separate musical genre. In the early-to-mid-70s, http://en.wikipedia.org/wiki/Italy - Italy was one the http://en.wikipedia.org/wiki/Europe - European countries most interested in this genre; many English bands such as http://en.wikipedia.org/wiki/Genesis_%28band%29 - Genesis , http://en.wikipedia.org/wiki/Van_der_Graaf_Generator - Van der Graaf Generator and http://en.wikipedia.org/wiki/Gentle_Giant - Gentle Giant were discovered by the Italian public before they had consolidated a fan base in their home country. Consequently, progressive Italian groups were prolific. Some received worldwide attention, such as http://en.wikipedia.org/wiki/Le_Orme - Le Orme , http://en.wikipedia.org/wiki/Premiata_Forneria_Marconi - Premiata Forneria Marconi , http://en.wikipedia.org/wiki/Area_%28band%29 - Area - International POPular Group and http://en.wikipedia.org/wiki/Banco_del_Mutuo_Soccorso - Banco del Mutuo Soccorso . Most of the bands, however, were mainly known inside Italy. As CD reissues started appearing and the Internet made information flow easier during the 1990s, the Italian bands were discovered and rediscovered by a number of progressive rock fans internationally. Reissues proved so successful that several recordings which were never released at the time received their first pressings on CD in the 1990s and 2000s. The "discovery" of Italian progressive rock by foreign fans also led to bands such as http://en.wikipedia.org/w/index.php?title=Celeste_%28band%29&action=edit&redlink=1 - Celeste being re-evaluated as core bands, despite the fact that they were virtually unknown in Italy at the time. The 1990s also saw a resurgence in bands performing progressive rock. The first of the well known bands to do so was Ezra Winston, but other groups such as http://en.wikipedia.org/w/index.php?title=Finisterre_%28band%29&action=edit&redlink=1 - Finisterre , http://en.wikipedia.org/wiki/Deus_Ex_Machina_%28band%29 - Deus Ex Machina , and http://en.wikipedia.org/wiki/Moongarden - Moongarden soon established themselves as well respected progressive rock acts. More recently http://en.wikipedia.org/w/index.php?title=La_Torre_dellAlchimista&action=edit&redlink=1 - La Torre dell'Alchimista and http://en.wikipedia.org/w/index.php?title=La_Maschera_di_Cera&action=edit&redlink=1 - La Maschera di Cera have carried on the Italian progressive rock tradition, sporting a very 1970's style.”  Source Wikipedia.