The Fourth Generation Languages where supposed to take over from the Third Generation Languages (C, C++, Pascal, Fortran, COBOL etc.) for higher level work (like business computing)
Many have been successful within niches, but Java was a brand new 3GL at a time when a lot of industry focus was on 4GLs.
I think sun had mindshare because they understood the nascent internet much better. From my perspective it was the first time I had seen a marketing effort of that scale behind a programming language.
Sun also supported SunSITE which was pretty much the best place to find FOSS at the time