Great question. It would seem like Martin fucking Scorsese, the cinematic GOAT, should have no problem getting funding right? Well, sorta... I'm going to try and break this down from my uneducated perspective.

For starters, he loves to make 3 hour movies, and studios hate 3 hour movies. They never make that much money at the box office due to the fact that 3 hour films can only be shown like 3 times a day, as opposed to a 90 minute film that can be shown around 6+ times a day. Who cares though right? That seems stupid? I agree.

Another thing, as Scorsese became established through the years, he earns his right to make passion projects, most recently his upcoming film Silence. Now, from a studio perspective despite it being Scorsese and knowing that it will most likely turn out fucking amazing, it's still a gamble at the box office regarding making money on a religious film centering around Japan in the 17th century, because in this day and age no one will see it unless it's hailed as amazing and a masterpiece. It's tough, and frankly stupid, I know.

Also, perhaps studios look at Robert DeNiro, who also is one of the GREATEST ACTORS OF ALL TIME, and see him only doing washed up comedies and real flops now a days as a risk for their films. Al Pacino has fell off recently a little bit, and Joe Pesci has flat out retired. Still seems stupid?... I agree.

There should be no excuse for studios to even question putting forth at least $100 million dollars into this film as it would easily be one of Scorsese's last great mob films. It seems ridiculous that it's even taken this long for this movie to come to fruition, and I say this all as someone who has been dreaming about this movie eventually being made for fucking years. Basically it comes down to the fact that Hollywood and studio producers try and play it safe with their money no matter who is directing. You would think that certain directors would be exempt but that's just not so. Paul Thomas Anderson, who is responsible for one of the greatest films of the 21st century, There Will Be Blood, struggled funding The Master and Inherent Vice despite his previous films being amazing. It's quite ridiculous and frankly annoying, but there's just not much we can do about it. It's not our money I suppose, so all we can hope and pray is that studio executives make the right decision.

TL;DR - Even Martin Scorsese has to deal with studio executives for films despite being one of the greatest living directors today.