Nationally TSUSA has many competitors...those behind the MA curtain don't typically have as many choices, especially ones as good as TSUSA is. Their proximity (I get orders quickly), plus free Prime shipping, plus "paying them back in karma" for telling MA to shove it are among the reasons I choose them over others.
Googling around, most sites don't even include them in a "top 5" or "top 10 ammo" vendor list; one site listed them as their "20th place choice." There's some myopia on this board as it's overwhelming MA-centric, and since they took on MA and will ship to MA they get a ton of exposure here that they wouldn't in free states. In those places they're "just another ammo site."
Targetsports is still a somewhat small fish nationally, but since they have a near monopoly on MA, I wonder what percentage of their sales come from New England. MA isn't huge population-wise or ownership-wise (which is why CA, for all its dumb laws, still gets places that play along in terms of compliance and stuff, since it's huge market regardless of the restrictions), but if they can corner the market, there's still money to be made.