Because the game wants you to feel like getting a Milotis is a reward and something special. If they just handed Feebas out, then they would lose meaning and Milotis would become no big deal. Would you rather think; "Meh, it's just a Milotis." or "OMG, A MILOTIS."
44. Getting a Latias/Latios in Ruby/Sapphire is a reward, Milotic however was a royal pain that wasn't worth the trouble it took to actually catch its pre-evolution much less maxing out its beauty stat. At least that problem was rectified in Platinum. The Regis in Ruby/Sapphire however were surprisingly good, thank god I had Regice on my team to counter my friend's Salamence and Rayquaza back when we battled.
That and I was one of the few people who had guys like Alakazam and Machamp back then too strangely enough.
48. Both are pretty good to use but I'd have to choose Alakazam due to faster speed and the fact that it hits so freaking hard too. But if you don't have anyone to trade with then Espeon isn't too bad either.
Abra can also easily be obtained from the GC, iirc. But don't ask me; I got a Dratini from there instead.
Gardevoir and Gallade are both so awesome~ Still loved how awesome my Gardevoir Roswell was in Emerald. He single-handedly pwned both Lucy and Norman. Still doesn't top my E4-pwning Bastiodon I used for the Challoween back in October.
58. Lost my Emerald game before I got to challenge any of the Frontier Brains in there, at least I got 4 out of 5 Frontier Symbols in Platinum, they wouldn't let me use my team of Charizard, Garchomp, and Electivire against that little asshat Thorton.
I had so many Pokes for the Battle Tower in Ruby it felt like a small army, my Gardevoir, Walrein, and Zangoose were definitely some of my best.