technically speaking the fees went down because of 3 main reasons which i will list in order of importance effect:
1. spam attack against bitcoin network stopped for whatever reason: maybe because SegWit was activated finally, maybe because BCash went away, or maybe because Dash and other altcoins that were spamming got dumped hard. but the spam attack stopped.

2. price became more stable, specially in the past 4 months. and without huge rises or huge falls there is always less transactions on the network.

3. with SegWit we got about 20% capacity increase (so far) which helped a lot and also some of the big services started batching their transactions which improved things a lot too.