Well let me put it this way, a terrorist attack is going to affect legislation going through capital hill (it almost affected immigration reform last week (and still might)) as well as the economy (confidence fairies and all that). You won't necessarily feel it going forward, or even for a while, but it will have an effect on things like security at events. Plus it's something out of the ordinary and terrorism is almost by definition a national issue. Plus you are right that it's really, really sexy. Where as the Texas explosion isn't really going to affect anything outside of Texas since the entire reason it happened was Texas's lax regulations.
For people living in Texas the Texas story will obviously be more important to them, where as the people living in New England will be more affected by Boston. Really only one of them will have national ramifications down the line. So that's the one they went with.
EDIT: Here's a link to the five factors that affect how news is covered. http://www.mediacollege.com/journalism/news/newsworthy.html It should basically explain my position. Boston wins in the significance category even though less people died due to the fact it was a terrorist bombing in a major city. You also need to remember that we didn't actually know the bombers were Muslim until recently, a fair amount of money was on them being white guys.
EDIT2: Also, remember that almost every major US city has a marathon so that makes Boston a bit more local. The question naturally becomes "If it happened there could it happen here?" So the national story becomes local as it will affect security at future outdoor events.
