Two things to try. Start TIA Portal by right-clicking on the icon and select "Run as administrator". The second thing is when starting TIA Portal, when the Simulator is first opened, leave it open (do not close it until TIA Portal is closed). One of those two things should work, but if not, reboot your computer and restart TIA Portal. This has happened to me in the past, and those are the steps I took to correct the issue.
As far as the "why", TIA Portal has to make a connection from its normal communication port (PG/PC interface) through a virtual port on the PLC (S7-PLCSIM). I like to use the PNIE interface, but sometimes, if it gives me problems, I switch the PG/PC interface to MPI to avoid contention for the ethernet port. I expect that sometimes there is an issue in capturing and reserving the PC's virtual card adapter for communicating with the simulator.