Wait... when were Janna and Soraka sololane contenders? Janna I can buy, maybe, but even that is quite lackluster unless the enemy chases your annoying kiting abilities. Not enough burst, even if you hit it all, and your AoE is situationally useful if you can land it, and Zephyr is single-target and drops your kiting performance, so pretty much a finisher at best. Her max combo does a whopping 545 (+115% AP) damage, of course excluding E AD buff, but come on, she's taking a solo lane. She needs reliable CC and burst to take up a full solo lane.
Then Soraka... Maybe before rework (I used to main skirmisher Soraka), but not now. E is near impossible to hit. W does miniscule self-heal only sololane unless ganked for, but she has situational CC, so ganks are semi-reliable anyway. Q can be harass, but is easy to dodge, and hard to follow up anyway. Even Q-E combo is pitifully weak to the point that the enemy will take it and charge you, and not care any unless you happen to have a jungler perfectly situated. Her max combo damage is 805 (+137.5% AP), most of which on E, which is risky to upgrade with its high cooldowns and low reliability. This also assumes PERFECT placement, which is quite difficult on her.
I don't see it. At least Karma has generally easy-hit CC, near-nonexistent cooldowns on Q, high burst with R, and shield/sustain to boot. Her solo capability far outscales Janna or Soraka, in my opinion. Her base combo is only 520 (+120% AP), but with an actual damaging ultimate that adds on 300-525 (+60-90% AP) and with a cooldown that creeps quite quickly in teamfights, so she can toss a lot of damage quite quickly, making her a far better solo laner than those two, at the very least.