This is possibly bigger than second opinions if it's an option for you. Part of it is just volume, the big national cancer hospitals see so many cases that they have super specialized departments & doctors to deal with much more niche areas of expertise.
Your local hospital might have "the skin cancer doctor" but MSK has ~20 sub categories of skin cancer with different doctors specializing in each. And so on for each area/type.
A relative of mine had a rare form of (very treatable) childhood cancer in the NYC metro area. It was diagnosed eventually by the local oncologist in the local hospital. His parents were ready to use the local hospital/doctors until the conversation turned to "and how many of these have you treated before and how did it go?". Sheepishly the doctor said "once and uhmmm, well.. it went ok".
So they sent the files (back in the physical paper files via mail era) to MSK and saw a super specialized treatment team that dealt with this regularly, successfully.