Q:

Front suspension on a cyclocross bike?

Is there a reason why no companies are manufacturing front suspension for cx bikes? 

Is it limited by the UCI rules?
Would weight mean it would offer no additional value?

I would like to know if a light weight, low travel (<80mm), skinny stanchion (<28mm) would make any difference to speed when off-roading. Comfort wise I imagine the difference would be huge. 
I have seen a picture of Lemond racing with a Rockshox fork so definitely been thought of in the past for road bikes. 

A:

Cyclocross courses are (generally) too smooth and relatively fast to warrant the use of suspension. Sure, it might be more comfortable, but the added weight would only slow you down and leave you behind  your competitors on a typical muddy course.

A:

Note: I don't do cyclocross, so this is mostly speculation.
I'd guess its primarily a weight issue, as well as the culture of the people who do cyclocross. I did find a manufacturer doing rear suspension though. 
This link is also interesting reading. The author first notes that suspension would make the ground contact better, but the weight or geometry changes seem to be problems. 
From that link, I'd like to quote from that article "UCI rules make no prohibition against suspension in cyclocross or road bikes, and road bikes with suspension forks have been used in UCI-sanctioned events before." In particular, it references this article on how suspension was used in the Paris-Roubaix and how the culture didn't appreciate suspension at first. They also note that current bike design focuses more on tube shaping and carbon fiber.  

