A mix of money, time and fan reaction.
Though fans who prefer the original language is still prevalent today, it was far, far louder back then. Fandom was also smaller back then too to help with this.
Money was a factor in both getting not just the rights, but setting up studios and hiring lyricists and vocalists for the job. It was also remember years back it mentioning that prices for proper licensing was higher here in the States than it is for other countries. Macross 7 is one such issue as Animeigo tried for it back in '98 and they had to pay two licenses, one for the animation and the other for the music. It was so steep they dropped away, and no one else touched it for the same reason.
That's not to say some companies tried to do something.
Though Viz did not dub their opening and closing songs they subtitled the songs in a way to sing-along if one wanted to. This did not do very well, as I remember a great deal of anger over that. This was the early to late nineties.
Geneon, Pioneer back then, actually did dub songs for Tenchi Muyo and El Hazard.
http://www.youtube.com/watch?v=UAVx53UHQ_Y
http://www.youtube.com/watch?v=Wy6ZKWq9Yug
http://www.youtube.com/watch?v=qAIOBuwdUXI
I'm of the opinion that the two english ops for El Hazard were superior than their originals. Unfortunately, in time they stopped doing so.
Funimation was the next one to dub their songs. What they did however, was actually employ a band to play the music while the vocalist sang. Take a look at those early dubs for YYH and some others like Blue Gender, and notice the music for the english songs are slightly different than the originals. I'm thinking that they had issues getting the right tracks for those at the time, as their more current dub songs, when they do dub them musically sound identical to the originals.
Of course, it does help when the actual band allows you to even have the song overseas, as TOKIO denied any right to play 7'O Clock News, the opening for Kodocha in any language for its domestic release. Funimation actually apologizes for that in a menu option in that first dvd. Similarly, L'Arc-en-Ciel's Fourth Ave Cafe was pulled from the english Rurouni Kenshin releases, but that was due to a band member being arrested.
With more and more opening and ending songs being used to push bands, its much harder to get not just the licenses, but the rights to dub them too, so that is one of the reasons why they'll dub some songs for one series and none for another.