It's called a reboot, they use it to start the series over with new design principles and modern technology. Calling it SimCity 5 would imply that it was successor tos SimCity 4 or 3 or 2000 etc... when it might not be.
To take an existing example, Star Trek (2009) is not a sequel or a prequel to Star Trek (1966-2008). It's a new piece of work rendering the old line as complete.
To take an existing example, Star Trek (2009) is not a sequel or a prequel to Star Trek (1966-2008). It's a new piece of work rendering the old line as complete.