The floo network is not very accessible in Hogwarts. Not every fireplace connects to it. There is a networking system in Ministry of Magic and to alter that, it do need time and careful planning. Snape and Karkaroff didn't know anything about the portkey.
Snape, Karkaroff and Crouch Jr. were present at the Quidditch field at that time. And it would be extremely suspicious if they disappeared without a trace and Harry ends up dead later.
Nobby and highsorcerer made excellent points about the reason for Crouch careful planning from the start;
1. getting Harry into Triwizard tourament
2. making sure Harry could be the first to reach the portkey
3. making sure no one else does...but Cedric did
All these to make sure no one suspect Voldemort is responsible for Harry's death.