Of the 22 songs on the Bombshell recording, 12 of them have Marilyn as the only singer or the continuous soloist in front of an ensemble/dance number, and 4 of them involve involve her in a duet. That's 16 numbers, most of which are heavy belting, high energy with lots of dancing, or both.
Eva is also listed as being in 16 numbers, but most of them are group numbers or duets. She effectively only has 4 real solo numbers, all of which are very demanding. Eva is a SUPER difficult role. Marilyn, as written, is quite literally an impossible role to do 8 times a week, and I'd argue even 6. I actually think the closest thing to Marilyn is Fanny Brice, who is in almost every single number in Funny Girl. I also think that's one of the reasons we haven't had a notable revival of Funny Girl anywhere on this side of the pond in forever.