Without exception, any online game will have its servers go down at some point or another. Updates, reworks, patches, DLC, holiday events, severe weather, and DDoS attacks can all cause a server to go down.
But sometimes servers go down for more ridiculous things, such as high number of players. A new game launch, in particular, can bring a server to its knees under the weight of a sudden influx of players.
Elite Dangerous has been around for years though, so by now the developers should know what to expect in terms of playerbase numbers and how much strain the servers need to be ready to handle. So we're hoping that when launch day rolls around for the PS4, those servers are already primed for the onslaught of new adventurers and won't collapse under the added pressure.
We'd much rather get to actually play the game on launch day, rather than ruefully watching Elite: Dangerous streams and wishing we could.