Well, maybe it's because Minecraft hasn't really finished "evolving" yet. There have been various updates adding a couple things here and there, but not really much to qualify as anything significant in the way of evolution. I enjoy Minecraft early on in the game, when you have to try to find a staple food source, get yourself some stone tools, and fight mobs that are kinda challenging.
However, the difficulty is the first thing to be thrown out the window, as diamond armor, enchants, potions, etc. remove any potential challenge that may have existed. The game is nice, and the beginning is fun, but the mid/endgame isn't really developed enough to still be fun. Many people often complained that certain suggestions made the game too easy, or that certain weapons (such as muskets/crossbows) would be overpowered, yet the game isn't really hard in the first place.
The problem for me is that once you have diamonds... Scratch that, once you have a dirt hut and a food source, you can't die. Mobs never try to break in, and aren't even that tough either, not like there are even that many of them anyway. The last time I played Minecraft I had diamonds within an hour, with a solid farm and a simple shelter set up as well, instantly lost interest already.
Quite a ways back, I used to check the forums frequently just to see if there was an update, and the potentially amazing things it brought. I was disappointed far more than I wish I had been. The combat is ridiculously boring, there are only two weapon types that appear the exact same (aside from colors), armor looks the exact same, etc. Sure you can still build, but I just don't even feel motivated at that point, it isn't necessary, and I don't feel like putting huge amounts of time into a structure that I don't even need to build.
On the outside, the game seems like a huge, never ending world with adventure and fun around every corner. What it turns out to be is absurdly boring mining, simple combat mechanics, and structures that appear the exact same as the last one you found. I definitely got my money's worth though, but just wish it had grown more than it has.
(To think I didn't even feel like commenting at first, but made all this. To clarify any potential issues, I do still think the game is good. However, it has the potential to be so much better than just good, but I won't be checking back too often if that ends up happening soon. MP is still decent if I actually feel like getting on Minecraft [(not PvP, just like hanging with friends and building a small town]).