I suspect that there are many practical reasons why you might not have a guard. For example they may get caught, or stuck in clothing or against straps.
However in this instance I believe that it is to do with the undertones of Japanese culture that can be seen within the Star Wars franchise films.1 There are plenty of examples of this dotted about online, but it's largely understood that it was influenced if not subtly by the films of Akira Kurosawa.2
With this in mind it's possible that the lack of guards could have been influenced by a cultural or artistic intention; Uesugi Kenshin3 believed that a true warrior doesn't need a tsuba (guard) so their katana didn't have them. Pretty unique for daitō (long swords) in Japanese history.
This is what I've always understood to be the reason, but I would imagine that there are much more detailed breakdown of why this could be the reason elsewhere.
Some of the other answers theorize some really interesting physics based explanations as to why slippage and therefore a 'guard' is not as large a concern with a light saber as you'd think. In Omegacron's answer he mentions about the lack of physical weight above the hilt, and as such the lack of opposition from forward momentum.
Consider that with traditional blades you have a metal mass above the hilt namely the blade, that you are trying to force into an object; either by a swipe or stabbing maneuver. As the two physical objects collide there is a counter-force to your initial expenditure.
For example a stabbing motion would cause the friction between your hand and the swords grip to become the focal point of the resistance met by the two objects colliding.
(Taking into account the lightsaber's plasma blade)
The lightsaber's plasma blade would indeed have a mass, but there are two considerations to take into account when calculating the factors of resistance from stabbing and swiping maneuvers.
For the most part the canon refers to the fact that a lightsaber could cut through virtually anything so in a duel between two individuals the only thing that could in theory provide any actual collision and therefore a counter-force is the opponents own lightsaber. This counter-force would not be the same as that of two metal swords clashing, as the plasma blade of a lightsaber is said to be maintained within its own magnetic field4, the two blades would likely experience a magnetic counter-force. A magnetic counter-force would not cause reverberations down the body of the hilt which would rule out one such force that necessitated guards on weapons.5
So with that in mind when it comes to a lightsabers' blade swiping, or stabbing at something that's made of organic matter, then it is quite likely that it would provide no resistance to the initial expenditure because either the 'beam' enters or passes through the object without much issue.
1 Japanese culture in star wars: Link 1, Link 2, Link 3
2 Wiki reference to Akira Kurosawa
3 Wiki reference to Uesugi Kenshin
4 Wooki link regarding plasma.
5 Very interesting information on impacts and motions of swords