evolution - Why do plants have green leaves and not red?

There are several parts to my answer.

First, evolution has selected the current system(s) over countless generations through natural selection. Natural selection depends on differences (major or minor) in the efficiency of various solutions (fitness) in the light (ho ho!) of the current environment. Here's where the solar energy spectrum is important as well as local environmental variables such as light absorption by water etc. as pointed out by another responder. After all that, what you have is what you have and that turns out to be (in the case of typical green plants), chlorophylls A and B and the "light" and "dark" reactions.

Second, how does this lead to green plants that appear green? Absorption of light is something that occurs at the atomic and molecular level and usually involves the energy state of particular electrons. The electrons in certain molecules are capable of moving from one energy level to another without leaving the atom or molecule. When energy of a certain level strikes the molecule, that energy is absorbed and one or more electrons move to a higher energy level in the molecule (conservation of energy). Those electrons with higher energy usually return to the "ground state" by emitting or transferring that energy. One way the energy can be emitted is as light in a process called fluorescence. The second law of thermodynamics (which makes it impossible to have perpetual motion machines) leads to the emission of light of lower energy and longer wave length. (n.b. wavelength (lambda) is inversely proportional to energy; long wavelength red light has less energy per photon than does short wavelength violet (ROYGBIV as seen in your ordinary rain bow)).

Anyway, chlorophylls A and B are complex organic molecules (C, H, O, N with a splash of Mg++) with a ring structure. You will find that a lot of organic molecules that absorb light (and fluoresce as well) have a ring structure in which electrons "resonate" by moving around the ring with ease. It is the resonance of the electrons that determine the absorption spectrum of a given molecule (among other things). Consult wikipedia article on chlorophyll for the absorption spectrum of the two chlorphylls. You will note that they absorb best at short wavelengths (blue,indigo,violet) as well as at the long wavelengths (red,orange,yellow) but not in the green. Since they don't absorb the green wavelengths, this is what is left over and this is what your eye perceives as the color of the leaf.

Finally, what happens to the energy from the solar spectrum that has been temporarily absorbed by the electrons of chlorophyll? Since its not part of the original question, I'll keep this short (apologies to plant physiologists out there). In the "light dependent reaction", the energetic electrons get transferred through a number of intermediate molecules to eventually "split" water into Oxygen and Hydrogen and generate energy-rich molecules of ATP and NADPH. The ATP and NADPH then are used to power the "light independent reaction" which takes CO2 and combines it with other molecules to create glucose. Note that this is how you get glucose (at least eventually in some form, vegan or not) to eat and oxygen to breath.

Take a look at what happens when you artificially uncouple the chlorophylls from the transfer system that leads to glucose synthesis. http://en.wikipedia.org/wiki/Chlorophyll_fluorescence Notice the color of the fluorescence under UV light!

Alternatives? Look at photosynthetic bacteria.