This is so because network speed/difficulty increases, therefore block difficulty increases too,    but per-block reward remains the same. You can see this in terms of total Monero network "speed"  (which is the combined hash power of all miners). For example, it increased twofold over the period from November 2017 (350 MH) to January 2018 (700+ MH). The network is self-regulating: network speed will grow until some miners consider mining not profitable enough, and in such case network speed will drop.