[Crystal_Ageha]

^ That's what I was gonna say. There are so many words in English compared to any other language. It really is wonderful when put to good use (though, people don't usually take advantage of our amazing possibilities, myself included, hah).
In Japanese, there are a lot of things you can do that you can't do in English... But like Jackie said, the fact that there are many words that mean many different things, or even kanji that can mean different things, it can get in the way. It's easier to just try to present it all in English.


But also, another thing I noticed is, almost all pop bands have pretty much all English titles; when they do use Japanese, it's usually something very simple (like "Ai no Uta" or something). This is probably because they want to prepare ahead, in case they get popular outside of Japan (which is definitely very likely, to at least some degree).
Artists who, on the other hand, aren't necessarily trying to fit into mainstream or don't have 'typical' music are more likely to go for many more Japanese titles.

For example, people like Ayu and Namie Amuro use English titles, because they both are pop stars who were both going for the pop star career, simple as that. On the other hand, somebody like Ringo Shiina - somebody who may technically be a "pop star" to some, but who definitely always went off to do her own thing in her music - uses pretty much all Japanese.
However, this shouldn't be used as a rule. Because, for example, somebody like Ai Otsuka - who really can't get any more pop star, hah - uses plenty of Japanese titles, or at least often originally writes them in katakana when they are English. And then, there's Dir en Grey. They are not mainstream, but I've seen more English from them than Japanese.


Another thing to consider is that, as the years go by, it's become much more common to use fully English titles, and to use more English in your songs. You can see that by just looking at the titles of older covers people do: even the most popular songs were often in Japanese, from what I've seen. (Not that there weren't English titles, but I've seen more Japanese, with the exception of anime themes.)