Feebas is the infamous, ugly fish that everyone hates. This useless peace of nature is, however, incredibly rare, and it evolves to such an incredibly beautiful, lustrous Pokemon that everyone wants desperately.
In the lake of Route 119, there were only two Pokemon that could appear. Carvanha and you-know-who. (If you really don't know who, it's Feebas genius)
Carvanha appeared the majority of the majority of the time. (The second mention of the word majority was intentional) while Feebas only appeared in 6 measly tiles of water, out of the approximate 270 tiles of water. That's only one tile in forty-five tiles. Just over a 2% chance.
What makes this even worse and even more time-consuming is that there was a chance that Feebas would not appear in all six tiles all the time. You had to fish there multiple times to see if Feebas was there. Painful.
Now, if you think you've checked all 270 tiles (which I doubt you have) you can always shuffle the tiles by going to that useless 'Trendy Saying' man in Dewford City. Talk to him and the locations of Feebas will shuffle. Turns out he's not all that useless after all.
I'm sorry about the wall of text, but I had the urge to say all that. xD
Hope I helped. :)
One Source, Two Source!
EDIT: According to the first source, there are 279 tiles. It was manually counted though. And it was my question. x3