Matt Brown is in a position to be established enough through his previous fight history to keep sponsors thanks to positive relationships built from back when the company was making walkout shirts, banners, and so forth. Newer fighters (less established or brand new) are less likely to keep previous sponsors as their biggest bargaining chip was the fact the sponsor would be seen come fight-time. Without that, their chip is essentially "People will hopefully remember me from the event, and so will start following me on social media, and I'll give you a push through advertising post-hoc". It shouldn't surprise you that the interest in paying for that is lower than having the assured commercialized clothing on a showcase.
As said before, Matt Brown is in a fortuitous position. And if you have a really really hard time believing that fighters are missing out on payment, try asking them. I know I've talked to quite a few here. Most are losing out. Others are smart enough businessmen (or have smart enough managers they pay for) and maintain a somewhat reasonable pay via sponsors.
I think it's important Sherdoggers and fans understand that not everyone is being outright destroyed by the Reebok deal. I think it's important to understand the Reebok deal COULD improve the quality of pay for fighters down the road via stronger corporate sponsors. I think it is equally important to note that the deal appears to have been rushed and it's caused a bit of a headache in the short term. Doesn't mean it's the devil;