Money.
It all comes down to what they think they can sell, how much it costs to develop and build, and what their return is. Demand is what drives supply.
+1
The Big Boys are just like any other company in the corporate world. They need to design, test, and manufacturer products that sell. And if you can't turn a profit at it, you will cease to exist. REVENUE (money coming in) - EXPENSES (money going out) = NET PROFIT. It's a battle to maximize REVENUE (charge more, increase volume) while reducing EXPENSES (cheaper components, cheaper sources, efficient manufacturing). Cheap out on a component and you could jeopardize quality, impacting revenues. On the other hand, manufacturing product that is expensive (complex build process, high priced components) will eat away at your revenue.
As an example, in the DIY world (Greenboy, Bill Fitzmaurice, etc...) you have several choices to make for your design: 2 way, 3 way, mid driver, tweeter, 2x12, 2x15, etc... You probably KNOW what you want and what your budget is, therefore making more informed choices. (Ok, maybe not everyone. ;-)) A corporation has to make all these decisions without really knowing 100%. Hopefully they've done some market research and don't charge in blindly. Therefore, they will tool up and build what has the best chance of maximizing their profit.
In my opinion, this is one of the reasons you see more 2 way designs on the market than 3 way. There doesn't seem to be a huge outcry for 3 ways and the 2 ways are just cheaper to make (fewer drivers and far simpler xovers in most cases).