Little software exists at present. It is difficult to implement sophisticated software on distributed system, because the software has to take care of many machines simultaneously for their interaction.
Because of users share many paths to the network; the communication is slower compare to a stand-alone system. Moreover, if the network underlying a distributed system saturates or goes down, then the distributed system will be effectively disabled thus negating most of the advantages of the distributed system.
In a distributed system, everyone can conveniently access data all over the system; people may equally be able to conveniently access data that they have no business looking at. In other words, security is often a problem. For data that must be kept secret at all costs, it is often preferable to have a dedicated, isolated stand-alone system that has no network connections to any other machines.