Because for a while they put a maximum on it instead.

At the time, netbooks were growing organically and could be had with 2 or 4 gigabytes un some cases, with Linux for less that the Windows version.

Microsoft countered this with cheaper versions of Windows that were limited to lower specs, such as 2 gb.

This was the so-called "starter edition" of XP and similar licensing programs for Vista.

The policies might have changed with Windows 10 (not sure), but the legacy remains.