Recall the logical consequences of the way Windows converts single-clicks into double-clicks. The double-click action is an extension of the single-click action.
In the case of context menus, the proposed double-right-click action is not an extension of the single-right-click, because the single-right-click displays a menu, whereas the double-right-click would go directly to the Properties dialog. Indeed, the way context menus are positioned on the screen specifically negates the possibility of double-right-click, for the context menu positions itself (under normal conditions) so that the click point is aligned with a dead spot on the menu. That way, when people who instinctively double-click everything do their double-right-click on an item, they get a context menu on the first right-click, and the second right-click is ignored. If the context menu were positioned so that the click point was aligned with an active location on the screen, then those habitual double-clickers would be accidentally invoking some context menu command when they really wanted to call up the context menu.
Now, if you decide to ignore the extension principle (and there are certainly lots of people who do), and if you decide that habitual double-clickers deserve to lose (and you're perfectly happy telling them that they're idiots who don't deserve the privilege of using a computer when they complain, "Your operating system is hard to use"), then you have to add reaction delays to single-clicks so you can perform separate single-click and double-click actions. Seeing as using right-click to display a context menu was not widely known prior to Windows 95, it would probably be even more confusing if a right-click on Windows 95 didn't do anything... only to have a menu appear a half second later after you had given up on the click and moved on.
The Alt+double-click (and equivalently, Alt+Enter) shortcuts were added as a concession, so that people whose lives wouldn't feel complete without a shortcut for calling up properties can finally die happy.