I was involved with Java applets when Java was in beta and one of the most annoying things was that JDK 1.0 did not come with serialization.
I would build applets that displayed content generated on the server and serialization-deserialization code could be 2/3 of the program. Most people had a slow net back then, so the download performance was a big deal.
JDK 1.1 put serialization in right away, but Netscape had already introduced it's own incompatible serializer, and there were delays in getting JDK 1.1, plus people didn't like Java serialization -- practically there was a year or two where you couldn't use it.
Applets had enough teething pains that people quickly came to the conclusion that they weren't worth the trouble, then Flash came along which came closer to what people wanted at the time.