With the exception of Captain Falcon and Ice Climbers, all the top 8 characters have had a player prove that they can consistently perform at the highest level (usually by being the undisputed best player at some point using that character). For example, Armada won every tournament he entered from GENESIS 2 (July 2011) to Apex 2013 with Peach, a character that is generally considered to be worse than Marth (although I forgot about his Young Link secondary for Hungrybox's Jigglypuff when I wrote this).
Fox: Mango
Falco: PPMD, Mango
Marth: Mew2King, Ken (if you want to count him since he's old school and no longer top level)
Sheik: Mew2King, Amsah (maybe)
Peach: Armada
Jigglypuff: Hungrybox, Mango (maybe)
You could could count Wobbles/Fly Amanita and Hax if you want, but they're not quite comparable in my opinion. Also ChuDat and Isai, but they're probably too old to be very relevant to the current character viability discussion.
If you don't like spacies, then you probably shouldn't play them; you will not be able to play to as much of your potential with characters that you don't like. For example, if you are good at combos and bad at the neutral game, then you would not perform well with a character like Jigglypuff (who would squander your combo ability and expose your poor neutral) even if you practiced just as much as you did for a character you are more suited to. You could overcome the deficit with practice, but there will always be areas which you are relatively good at and relatively bad at regardless of your overall skill level; you will be better with a character you are naturally suited to than one you are not.
There is not only talent to consider, but fun as well; could you bear to play for hours every day with a character you don't even like? Especially spacies since you're going to be spending much of that time grinding techskill, more so than most other characters. Just be glad that you like good characters and not Kirby or something.
I like to represent it in terms of (arbitrary) numbers:
Your "default" performance is worth 100 units
Fox is worth 50 units
Marth is only worth 40 units since he's a worse character
Because you prefer Marth you gain an extra 20 units from playing him, while you only gain 5 from Fox because you don't like him and aren't as naturally talented with him.
Overall you have 160 units of performance with Marth, while you only have 155 with Fox.
All of this is a huge oversimplification since there are so many factors at play, but the principle still applies.
By the way, Mew2King has said that he thinks that Falco is the best character, but he doesn't play him because his other characters suit his style better. He has also often expressed worries that Marth and Sheik are too limited in high level play compared to the spacies, but he doesn't want to switch to exclusively maining Fox for the same reason. Axe has said that Pikachu is his best character even though his Falco is really good too; he might be able to win more if he mained Falco instead, but that's assuming that he will be just as good with Falco as he is with Pikachu, which isn't necessarily true. Think of almost any non-spacie player, and wonder why they don't play spacies and yet still manage to do well. In a hypothetical situation where they have to play spacies for some reason, how many do you think would be at least as good as they are now/be motivated enough to keep playing competitively?
There don't seem to be as many high level not-spacies player as there used to be, but that's not necessarily always going to be the case; characters are constantly falling in and out of popularity (even if the speed at which this occurs varies).
