goonies
a) Because far into development of the game some MSX game developers thought "what's this goonies movie about anyway?"
or
b) Because Konami knew that, 21 years later, CrappySoft would make a nice MSXdev entry because of it 
why the goonies on msx is so different when compared with other platforms?
Well, the PC version (Goonies 20th Anniversary Edition) looks a lot like the MSX version.
Weird! In the MSX version, does the player control Mr. Sloth?
If the manual says so, he doesn´t looks like Sloth in the movie! 
In the MSX version, you also play with Sloth (according to the manual at least), but apparently the game uses coder's art in stead of real graphics, so it doesn't look like Sloth at all 
Weird! In the MSX version, does the player control Mr. Sloth?
If the manual says so, he doesn´t looks like Sloth in the movie!
Yes, in the MSX version it is Sloth (I must admit I was surprised as well, I always thought the player was the older brother).
Indeed, the MSX player doesn't look like Sloth, but since the manual says it is Sloth, the Braingames team decided to do
what Konami wanted to do in first place, but couldn't do, whatever the reason was.