[edwinnathaniel]

Every single thing that the other people have replied plus 2 things:

1) Tons of experienced and battle-tested people in the ecosystems

2) Standards that keep improving

It's a dual-edge sword thingie, on one hand standards are good for stability but bad because sometimes it slow innovation down. Having said that, the Java ecosystems typically have one strong alternative outside the standard so your choices, in most cases, come down to 2: use what comes from SUN/Oracle or use the leading open source alternative solution. Simple.