It's because regardless of what ever speed Ono reaches, majority of his checks, not to be confused with counters, after still faster then him after 1 DD. So, you invest in minimum Speed EV's which allows you to outspeed them, i.e. 80 speed EV's, i.e. last check is Chomp. That's where that so called "outspeed garchomp" garbage is coming from. If your chomp happens to be your only check left to Ono to prevent his late game sweep, well then your fucked, because bulky sets will, and CAN, survive his Outrage essentially laughing in your face for being baited into allowing him to setup 2 DD's. He can't get 2 DD's without support any earlier then mid-late game. Any other time it's not worth it. Prepared or not, when your checks are done, you can call it game.
And in no retrospec is it useless to go for two DD's when that's what can seal the deal for Ono 90% of the time. When you can get 2, get 2. Secure the victory. If you only need one, meaning anything they have left can't outspeed you, or can live through +1 damage, go for the +1.
Edit- Don't want to sound harsh or anything, but still. If your not running a bulky set, and running the min/max/standard sweeper Ono, +1 should be fine if it's late game (no checks and all).