> 1) When you get older math got harder.
> 2) When math got more abstract it got harder. Blame Grothendieck.
> 3) When math stopped being tied to the real work it got harder. Blame Hardy.
> 4) Math has always been hard. We NOW understand some of the older math better so it seems easy to us, but it wasn't at the time.
> 5) With the web and more people working in math, new results come out faster so its harder to keep up.
> 6) All fields of math have a period of time when they are easy, at the beginning, and then as the low-hanging fruit gets picked it gets harder and harder. So if a NEW branch was started it might initially be easy. Counterthought- even a new branch might be hard now since it can draw on so much prior math. Also, the low hanging fruit may be picked rather quickly.
Personally I think it is mostly 2 and 3. And a lot of it has to do with how math is communicated. The very abstract way that math textbooks use to communicate is not how most mathematicians think when solving problems. A lot of things I struggled to learn from books became much more understandable once someone gave me the little insight I needed to make sense of it in conversation.