Because 99% of Windows Perl users don't have C compilers and CPAN is built around using your C compiler to install stuff.
Ok, this is true, but all you need to work with CPAN is a functional 'make' and 'install', correct? Does CPAN depend in on the C preprocessor or somesuch? There are win32 ports of GNU make, and install shouldn't be too tough...just seems silly not to support CPAN. Maybe licensing issues were a problem (GPL, whatever), but they could easily have rolled their own instead of writing ppm...heck, I've written my own dependancy graph traversal system.
Heh, this is something of a pet peeve for me, seeing as I've been shackled to a perl on Win32 project for a month or so now. Well, that and the complete insanity that is the Win32::PerfLib structures. Trust me, stay far, far away, monks. =)
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