In order to keep it cheap. MP3 is not an open format, for example the royalties on a player (if you develop your own MP3 decoder in-house) is $0.75 per unit. Multiply that by 2.2 million (and that's just the number of DSi sales up to Oct 2009) and you get an additional $1.6 million Nintendo has to pay to make the DSi units. Add on the licensing cost for other formats and it gets even worse.pachura said:E.g., Sony has made PSP a portable media center, while DSi's music player cannot even play fucking MP3s.
Sony's also an "entertainment" company and makes other multimedia products, so likely they're able to work out way cheaper deals with patent-controlling companies.