because because because
someone will from time to time come up with a new format - generally a compression format - but also DRM encrypyion
mp3 is good - but the main reason it's used is that it's used - and there isn't a good reason to go to the expense of changing it (think of the cost of changing out mp3 players ... I wouldn't do it)
http://www.fileinfo.com/filetypes/audio
128 or above is a good bit rate - and that's what the commercial people use (higher = better = bigger)
in terms of mp3 players - it's a trade off - more tracks = higher compression = worse quality.
http://www.nch.com.au/switch/index.html
but most mp3 players are used with tiny tinny in ear phones - on planes, trains and in the street - so only a dog could yell the difference.
there are technically better formats - but the players aren't available for more than 2 or three other "flvours"