Well, the 3d3d orbitals drop in energy as Z_(eff)Zeff increases across the 1st-row transition metals:
By the time we get to copper (Z = 29Z=29), the 3d3d is apparently low enough in energy that it is more energetically favorable to pair a 3d3d electron than a 4s4s electron. So, 3d^10 4s^13d104s1 is more stable for "Cu"Cu than 3d^9 4s^23d94s2...
As for why "Cu"^(2+)Cu2+ is more prevalent than "Cu"^(+)Cu+, that's a mystery to me. Apparently, "Cu"^(+)Cu+ is less stable than "Cu"^(2+)Cu2+ in aqueous solution.
2"Cu"^(+)(aq) -> "Cu"(s) + "Cu"^(2+)(aq)2Cu+(aq)→Cu(s)+Cu2+(aq)
And it turns out that during the hydration of "Cu"^(+)Cu+, forming "Cu"^(2+)Cu2+ by releasing one more electron to reduce another "Cu"^(+)Cu+ allows the release of excess hydration energy.