It's been a standard feature in Intel/AMD cpus since 1995.
closed as too localized by Shane Madden, Iain Oct 7 '12 at 6:57
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It's off topic and will be closed but I will answer you no matter what: because there were no laptops, absolutely none, back then that could've used more than 4GB. The first Pentium M platform using the 855 chipset was using DDR1 RAM. The largest capacity of DDR SO-DIMM (at least widely marketed) was 1GB. Even a portable workstation has only 4 slots. So why bother?
Edit: To quote Wikipedia:
And, indeed, Sonoma (mobile 915) used DDR2 which had larger capacity modules later, even 4GB ones.