When I knew bitcoin, I felt it was complicated and difficult to use. The first time I was really depressed and did not use it anymore but gradually got friends help, I knew more about it.
How is this related to OP's question regarding block difficulty?
Anyway, network hashes vary greatly which can be caused by many things, for example as prices change people may migrate to another coin to mine which would affect total hash-rate. It is very difficult to predict and adjust realtime, and by using historical data you could only guess these conditions.
the difficult for me is that people is hard to predict
