Well from my perspective I won't say those 5 are the top 5 now and forever. What I will say is id be surprised, barring significant nerfs, to see them outside the general top tier.
We can build some connections here based on traits we see are successful. For instance in Melee strong mobility is king. In brawl it was spacing tools. Smash 4 we see mobility is very important, long combo strings, and perhaps a projectile or other decent spacing tool for good measure.
This doesn't exclude others from being great but they have to carve out a different niche. For instance Jiggs/peach in Melee acting as an impenetrable fortress, or Wario in brawl dancing around his opponents. You even have characters like brawl diddy who didn't have the degree of snake/olimar/MK spacing tools but could make use of his tools (bananas) in a unique way to essentially do the same thing.
Part of this includes growth potential. Characters like fox don't have the raw combos of other characters, but as players become more efficient can extend them for a very long time. Improvement in uses with ground mobility I expect will greatly benefit pika and fox (in particular) and the other big 5 more then other characters.
If the meta shifts and we begin to see that mobility maybe isn't so important or spacing tools become more significant in relation to other traits, then characters like Rosalina and mario perhaps fit more smoothly into top tier. But for now they'll have to carve their own niche against the more predominant meta.
Meta:
Partial meta:
Anti meta 1:
Anti meta 2:
Etc.
I know anti-met usually means something that counters the meta, you can still look at it that way and then judge its ability to succeed
