There are probably posters who will give a far better answer than me, but the only reasons I can think of are 1) they don't want to take on the obligation without having earned the original commission, and 2) they are willing to take on those new commitments because of the commissions, but also so that it will be a broker-dealer agreement with the MM of their choice, not from the big brokerage firm.
I could be totally out of line, because it's really just a guess.
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