This is probably because they got a new composer for DAI. Inon Zur was the lead composer for DAO and DA2, whereas Trevor Morris succeeded him for DAI.
I was actually a little worried about this when I saw this before DAI came out, and felt sadly vindicated afterwards. I kind of agree with the OP that DAI lacked the "OOMPH!" that DAO had. Apart from the SAD MUSIC of Alexius' Bad Future and Dorian's personal quest, and the music that plays when everyone first discovers Skyhold, I thought it was just eh. Okay, but not too evocative or memorable.
THAT SAID, I think Morris really upped his game for the Trespasser DLC. I thought THAT music was consistently very powerful, passionate, and evocative. I feel like the music aided the tone of Trespasser--the feeling of urgency and danger the DLC story showed ("OMG, we're being attacked by psycho Qunari!" "OMG, my mark is KILLING ME!", "OMG, we HAVE to beat the clock before it's too late!"), but could also be really sad and wistful when it needed to be, especially when the Inquisitor confronts Solas. I thought that was so masterfully handled.
If the music of the next game is like the music of Trespasser (or, for whatever reason they rehire Inon Zur), I'll be happy.