The logic behind concluding there is a 20% chance we are living in a computer simulation is based on subjective probability.
-One possibility for humanity's future is we destroy ourselves before we become sufficiently technologically advanced
- another possibility is that we acquire the technology, but ethical considerations prevent us from creating this kind of simulation
- another possibility is that we just lose interest is creating a simulation
- another possibility is that there are just technical limitations to creating this kind of simulation.
- The last possibility is that we achieve a sufficient level of technological ability to create this kind of simulation, and decide to create computer worlds.
Not having enough information to state which possiblity is more probable, we weight them equally, and conclude the odds are 1 out of 5 = 20% chance we are living in a computer simulation.