From the Kritzkast interview with Robin Walker:
One of the other things we tried was a secondary weapon that instantly teleported the Engineer to his teleport exit. So Engineers could leave their sentrygun for a bit to skirmish or collect metal, and be able to immediately teleport back to the sentrygun if something bad happened. It did work at achieving those goals, but we didn’t like the side effects. Teleporters stopped being much of a team focused tool, with Engineers placing them in places that made sense for their personal use, and not necessarily for the team. We also felt it was too easy for Engineers, almost eliminating the risk inside the decision of whether they should leave their nest to grab some resources. Both of these were solvable issues, but while testing this we found another idea that played much better, attacked the same problem of Engineers being rewarded for moving out from their nest occasionally, and had lots of other interesting applications.Ah well. It was worth a suggestion.