I know they think two more tests before EM-2 sounds impressive. But EM-2 is currently scheduled for 2023, right? So two tests in 6 years? Seems like a pretty hectic testing pace. 
Quite.
Boeing and SpaceX have tested their life savers LAS engines numeroes times without all this public hubbub. For some reason either NASA or it's contractors (or both) find it necessary to turn SLS-related tests into major public events.
Yep. Two possibilities for these tests:
A) Pure PR events (but really, two-three tests are sufficient??)
B) These tests are so expensive they can only afford to run them once every few years (and if your tests are that expensive, that might be a clue that your system is going to be too expensive)
But hey, we get to see Apollo redux!
C. They changed the fuel grain of the LAS rocket so it produces less thrust, and that needs to be tested and qualified. Also these are temperature qualification tests, so they need to be held at different times of the year so they can be fired when the ambient temperature is hot / cold so the temperature conditioning of the rocket doesn't degrade. They also measured the acoustic levels produced by the blast of the rocket, as well as the flight avionics.
Also, all the test fires for the LAS (which also includes the QM fires for the attitude and jettison motors) need to be completed prior to the Ascent Abort test, which is scheduled for late 2019, but that may be moved up to early 2019.