First, because chemistry isn't particularly predictive. There are a large number of different reactions possible for most compounds, all leading to different products, and very small changes in reaction conditions can lead to large changes in the outcomes. Then again, you have to consider the potential reactivity of all the products when exposed to the same reaction conditions. You have to specifiy the starting conditions thoroughly to restrict the problem to a small number of outcomes, and with that type of restriction, if the reaction hasn't already been run, predictions of products usually aren't very useful.
I am aware of two types of solutions that have been studied. One is essentially a database miner - you enter your starting material and a limited set of conditions (other reagents, temperature, etc.) and the program tells you the products that have been attained under those conditions. Reasonable access to massive reaction databases like Reaccs and SciFinder have made this approach obsolete. Why ask a program what sort of reactions have been found for a compound rather than simply searching the reactions for yourself?
The second is an expert system approach - you enter your starting material, tell it what type of reaction you want it to undergo (substitution, elimination, reduction, elimination, etc.) and it tells you the products you can expect. There is some utility for these if they can incorporate enough geometry to tell you when your system is overconstrained to undergo reaction or if certain places are unavailable as reaction sites, or even determining which of a number of possible reaction sites would be most likely to react under different circumstances. For the most part, though, they are only useful for people taking organic chemistry exams. You still have to come up with the conditions and run the reactions to see if they work, and again, you can get the same information from SciFinder or Reaccs by choosing your substructures properly.
The second reason is that this really isn't the way most people approach chemistry. Chemical research is usually directed to a targeted compound and finding reactions that will lead to the compound, rather than trying to predict the ways a compound will react to other reagents. You already have a pretty good idea of how the compound would react to those reagents, or you wouldn't have been looking at that compound to begin with. Maybe that was a little obscure? See if this helps - you are asking about a program that would allow you to input a compound A and some reaction conditions B and predict what compounds C would be formed. Usually in research, however, you are more interested in designing a compound C and trying to determine what starting materials A and reaction conditions B would be reguired to generate that compound. Very rarely is there call for taking a compound A, trying a bunch of reaction conditions B on it, and seeing what sorts of final compounds can be formed - you almost always know what final compound you want, and the work is in trying to find the reaction conditions and starting materials to make it as efficiently as possible.