For those who ordered more than one colour (most of us), sending each order off as it reaches 500 could mean 5 separate shipments to the GH members, and potentially 5 separate shipping charges.
Conversely waiting until all colours have reached 500 would increase, at least in the short term, the workload of the dedicated GB organiser.
So it might come down to more expensive shipping for everyone, or more work for the GB organiser.
Perhaps the needs of the many outweigh the needs of the few.