Because anime fans will worship anything japanaysee. Jap OST sounds like carnival clown music. Voice acting is shit too.
Bruce Faulconer is way better, the Japanese OST sounds like circus music. People just always assume anything original is better by default.
Being more precise, it sounds like most 70's Saturday morning cartoon soundtracks from Japan (60's for America), including a lot of the very same ones that the same composer worked on, like Grendizer:
Like the DB tracks, they feature an overuse of scales like F Minor but since this one was made in the 70's it's acceptable and therefore makes it better than DB's even though they mostly sound similar. Same goes for works like the original Getter Robo, Gaiking, Daimos, Danguard Ace, etc but as opposed to DB and Z he did compose OP's and ED's for those shows.
And as an aside, I'd prefer it if we forgot about the Faulconer soundtracks for a moment as many other circumstances hinder them as well, but rather if we focused on what the BGM standard was in Japan during the 80's and 90's. E.g. literally every Gundam series that was made in those decades, Saint Seiya, FotNS, Ideon, Laserion, YYH, the Brave and Eldran series, most works by Sunrise and other studios, etc.
Content from the 80's:
And this is just the tip of the iceberg for this decade, too.
Content from the 90's:
And this is to say nothing of all the more popular series besides DB that came out then. If you want to know the titles, just ask me.
Also, the channel I took the most content from uploads only excellent soundtracks from shows that came out in the 80's and 90's, and only one of them is composed by Shunsuke Kikuchi. It isn't even from Dragon Ball either, meanwhile you find stuff like Saint Seiya on there.
If you've noticed, a lot of these do orchestral soundtracks just like the original DBZ OST does, just better. And they can incorporate more modern elements as well.
In light of all this, DB's original OST doesn't even enter the top 20 (or even 30) anime OST's of the 80's or 90's if one were to rank them after having seen enough. Not even the movies can save the show.
@rukelnikovftw: Let's just look at the OST on its own first, then compare it to the same composer's works during the 70's, and then to the works of other composers. We're not considering any American replacement scores here, just other Japanese OST's from that time period. Also, I meant that only the OP's, ED's, and other vocal themes as well as tracks not composed by Kikuchi are the ones I like. Kikuchi's compositions I don't like as much since they're rather formulaic and sound like his works 2 decades earlier. Not only that, you can drop them on certain other Kikuchi soundtracks and you'd be hard-pressed to notice a difference. They're not unique enough to DB.