The same reason the Mac is a poor platform for many Internet technologies.
The vast majority of Internet technologies were and are developed on Unix,
primarily because Unix has superb development and networking tools, as well
as for historical reasons.
There are many features of the Unix environment that have no analog in the
Mac environment. The result is that when technologies are ported from Unix
to Mac, some features are either 1) implemented by trying to work around the
limitations of the MacOS, or 2) not implemented at all. In the case of (1),
the resulting feature generally either doesn't work very well, or doesn't
support the full functionality found in the Unix environment.
The Windows 3.1 environment is nearly as bad as Mac in this respect even
though DOS has a few of the basic features of Unix. Windows 95 implements
most of the architectural features of Windows NT. Windows NT is pretty
compatible with Unix in terms of OS features and supports POSIX. The only
real difference that I can think of off the top of my head is that NT uses
threads instead of forking, which occasionally causes annoyances when
porting software (although threads are architecturally superior).
So to answer your original question, they had to do quite a bit of wrangling
to get Java to work on the Mac. As far as Java platforms are concerned both
Unix and NT have solid environments, and perform equally well. The one area
of Java where NT and Mac generally outperform Unix is in the interface
implementation (java.awt.*). The Unix interface implementation tends to run
really slow. This is mostly the fault of the X-Window system, which is kind
of slow anyway compared to the more tightly bound interfaces used by NT and Mac.
Hopefully this answered your question (and a lot more). You can use the Mac
as a Java platform, but Java will run better under NT, Unix, and to a lesser
extent Windows 95.
-James Rogers
jamesr@best.com