I have been on many Math PhD thesis defense's as the Dean's Representative. This means I don't have to understand the work, just make sure the rules are followed. I've done this for a while and I used to understand some of it but now there are times I understand literally none of it. As a result, when the student leaves the room and we talk among ourselves I ask
When did Math get so hard?
I mean it as a statement and maybe a joke, but I decided to email various people and ask for a serious answer. Here are some thoughts of mine and others
1) When you get older math got harder. Lance blogged on this here
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.