Because of the following:
Bushmaster is expensive for what you get.
They don't offer much variety (thin barrels, faster twists)
The internet has taught hundreds of thousands to build their own.
A lot of people are just building their own rifles with the parts and configuration they want. It usually comes in cheaper than Bushmaster, with a minimum of equal parts quality or usually higher.
The market has changed. There are so many choices these days in barrel profile and twist rate. Not to mention midlengths too. There are so many buttstocks, rails and options that for many people, it makes no sense at all to buy a complete rifle only to break it down and replace things.
DPMS gets more discussion because they appeal to a more value-oriented consumer base. If folks can't afford the higher dollar stuff, DPMS provides good options for less money. However, Bushmaster's prices are higher than or competitive to building rifles of equal or higher quality with specific parts tailored to the user's demands. If someone can't spend $900 or more on a rifle, chances are they aren't going to buy a $300 rail or a $400 optic or a $200 stock. They are likely to keep the rifle in standard configuration which means DPMS is fine for their purpose.
Bushmaster is still stuck in the past. HBAR's are starting to die off due to other choices and information on the internet. I've always hated them. Why should I lug around another 1.5lbs or more for NOTHING. For zero benefit. People also want the 1/8 or 1/7 more and more. More people handloading, and with Black Hills and others offering heavy loads it has increased demand.
The people who can spend $900 or more on a rifle are starting to take a serious look at medium to lightweight profile barrels, midlengths, and faster twists.
