Well, the whole deal with the "Age of Enlightenment" is that it is, overall, a happy period. Life is much less shitty than it used to be (vs the middle ages or even the renaissance), scientific discoveries are made at a rapid speed, and the middle class is getting very wealth (again, compared to the past). The music reflects that with all the light-hearted melodies, major keys and the I-IV-V moves.
On the other hand, Baroque music especially by Bach and Handel is ungodly complicated, and, well, people can't understand that. The word "Baroque" itself means a deformed pearl, which is not a praise at all. It means that people think the music is too over the top, too complicated, and a bit unnecessary. A nice little early Haydn string quartet is far more charming, at least to their ears.
P.S. still, a lot of Mozart (e.g. the d minor and c minor piano concerto; Symphony No.40 and 41) and Haydn (Symphony in f-sharp minor, A major "Fire", C major fantasy) is anything but bland.