1) Because they are basically the same system. The faster CPU was only used in a very small number of games. It was probably cheaper for Nintendo to go with the new CPU which is why they put it in there. Also the CPU could run at "normal" speed (same as Gameboy) or "2x" speed.
2) Again, they are basically the same system. The CPU could run faster but the Gameboy Color is still a Gameboy... in color. You could either play a game in black and white on an old Gameboy or play the same game in color on a GBC. Also, the Gameboy couldn't display 4 different colors, just 4 different shades of green (or grey, depending on the model).
Think of it as a DS and a DSi: The DSi has a faster CPU and more RAM but they are used to play the same games. You could, in theory, develop a game that requires the extra CPU power of the DSi but that would severely decrease the number of potential customers. The same was true with the GB and GBC which is why a lot of developers decided to make their games playable on both devices, the only difference being color.
Also, if I remember correctly, it was not "some" but "most" games that could be played on both models. The Wario Land games could, as well as the Pokemon games (all but the "Crystal" version) and even the "DX" games I think.