There's only so much cash to go around for each book.
And not a lot of money at all to spread so thinly when you account for how many individuals and services are required to make a printing of 30 thousand (PLUS!!) books.
Different projects have different projected potential revenue, which allows for greater or less initial capital investment.
Unless Morrison "demands" someone specifically, the more money they pay Morrison, because he can ask for premium dollars... The less the rest of the poor shlubs not on a fixed rate of pay can get for pencilling, inking, editing, and then a return for their stockholders.
With obvious exceptions, books using high quality expensive talent like Morrison, can't afford "established" artists with "reputations" no matter their skill levels, because they won't work for the slice of pie left over after Morrison has Gorged himself...
This works in reverse too.
It's why the "best" artists always have shitty writers who are often themselves.