Hello.. I`m sorry if my english is bad.
Why did composers compose their songs (for piano) in C minor, or B Major, etc.
Why didn`t they compose their song only in C major? It is easier, isn`t it?

using a minor key has a different effect than a major key. the minor key has a dark and moody tone. while a major key has a bright happy tone

I know that. But,...
Why did they use E Major, B major,etc.(Major scale), and they didn`t used C Major?
Why did they use F#minor, G minor, etc.(minor scale), and they didn`t used in A minor?
Why???

Some composers feel that different keys have a different effect. I agree I can`t hear much difference myself but Beethoven`s favourite key was C minor I think. Chopin also believed that C major wasn`t actually the easiest, he thought B major was because when you play the scale it fits the shape of the fingers better.

Music would be oppressively dull if it were all in the same key.

Most music of any sophistication and any length modulates any way, so even if all pieces were to begin and end in C, they would deviate greatly in the course of getting from C to C.

When deciding what key in which to put a piece composers take into account various technical considerations, the best ranges of the instruments involved and fingering difficulties (which vary from instrument to instrument), and so on, or they go with their subjective intuitive conception of the key. Most musicians conceive of sharp keys as bright and flat keys as black, and some go much further. I see in my mind`s eye C# major as a deep velvet red and E-minor as a greenish yellow (a color I call "electric yellow").

There is, however, a sense in which all music was once "written in C". This was during the modal period, before the major-minor dichotomy, of course, but there were at least no written sharps and flats. In this period notated pitches were not absolute, they functioned rather like solfeggio symbols in this respect. Musica ficta foreshadowed the collapse of this system.