They stopped selling other brands because Amazon in the US is getting all the sales for the main brands. As MMAWarehouse is a reseller, they don't have the margin to compete with Amazon, and if they decide to sell in Amazon, they must pay a 15% fee, so there is still less margin.
In the US the problem for multi brand shops is going to be worst and worst due to Amazon.
They decide to promote their own brand because of this, and this way they can also sell in Amazon (as they do) because Amazon can't compete with their own brand.