It is the fact that she can poke , harass, make mistakes and still get away which is why they're nerfing her survivability. Not that hard to understand is it?
Its more the fact she has worse stats than melee minion. In perspective if they were just attacking each other and both had the same base AD. Sona would survive with 1.3 HP. Now look at it another way there are a few champions that can 2-3 shot melee minions at level 2. So with the same stats sona can be easily killed. Her abilities keep her alive.