You're right, Jace--many movies have scenes "left of the cutting room floor." In the case of GF: Coppola was hired with orders from Paramount to bring the movie in under 90 minutes--and itran much longer, even after extensive editing. Coppola got cart blanche for GFII, and it ran even longer than GF, and needed more editing. Those scenes got valuable after GF and II became the most iconic movies since "Gone With the Wind," which is why we have access to them--and why Paramount gets us to shell out big bucks for boxed sets that include those deletted scenes. "Mean Streets" is, IMO, Scorsese's masterpiece, but it doesn't have the same global iconic status as GF and II. Here's a link to a fairly interesting commentary about its production:

https://www.google.com/search?q=mean+streets+deleted+scenes&oq=mean+streets+deleted+scenes&aqs=chrome..69i57j33i160l2.7757j0j4&sourceid=chrome&ie=UTF-8#fpstate=ive&vld=cid:1265a235,vid:Bxpxg_E9x-U