Is this a serious question, or a rhetorical one that you already know the answer to but are asking aloud just because?
Mattel has access to mass retail presence and a shit ton of money. Companies like DCC (which DC owns as their in-house production arm), Mezco, and Mafex aren't likely to wind up on store shelves due to myriad factors, not least of which is the fact that due to company size there's no way for them to produce the volume that Mattel can.
So if you were DC, what would look more appealing to you:
1. An established company with massive production means, a deep financial reserve that can always outbid the competition for contracts while still maintain profit gains, a history of on-time shipments to retail shops that perfectly coincide with media releases, and a lock on getting product on store shelves thanks to business relations and thus ensuring larger returns on product percentage
2. Companies that are 1/20th as large as Mattel (or smaller), with limited factory access and production means, an established and well known history of massive and frequent delays on even relatively small batches of figures (in the case of Mezco and DCC), smaller pools of money to lower their bid price to acceptable and competitive levels, and zero experience with establishing relationships between themselves and regional/national buyers representing mass retail stores
Quality means nothing when you know you're already going to sell hundreds of thousands of units per year on shit figures.