I respectfully disagree. The large number of fleet boats doesn't drive away donations and support. How many donations and support a museum sub gets is ENTIRELY dependent on its MANAGEMENT. How well do they run things to make their ship attractive to potential donors? How aggressively do they go out into the community soliciting donations? One fleet boat is hours away from another so its not like any are competing for donations in the same region.
The U-505 is where she is and displayed the way she is because the Museum of Science & Industry is a known, respected quantity in its region if not the country. Its board members are aggressive in generating support, especially among large corporations and foundations. Any fleet sub museum can do the same but it takes time, clean financial books and a lot of handshakes.