I think Brent is right - Mexico provided just the right amount of haziness to the situation.
It was kind of a black hole of information. Now matter how many Mexicans were killed - the only information that would have come streaming out of Mexico is that American guns were showing up at horrific crime scenes in huge numbers. I think it would have been impossible to get further visibility into that.
Tailor made for using the information as fuel to both back up the Presidents original erroneous statements - and make the case for stricter gun control.
But, when agent Terry was shot and killed - the dynamics changed. That was the tipping point for BATFE agents that had been frustrated with an operation that didn't seem to make sense to them anyway. Now BATFE agents who had been sounding the alarm on the dangers of the plan were going to make sure that it wasn't swept under the carpet - they blew the whistle. And they knew that the firearms could be traced.
Totally different from just a bunch of foreign drug gangs shooting each other in another country.
Since it happend on our side of the border - the information could be found, exposed and reported on.