Short answer: Balance
Long answer: If you allow Syndra Orbs to be destroyed, her power would have to go up WAY too much to compensate, as this would make her much more unreliable. Unless you make the mechanic of destroying them so hard to use that it's not even worth it to try and destroy them (for example, small hitbox and tanky as fuck), hence making the mechanic useless and just wasting everyone's time.
Heimer has his rockets and bombs. GP has his Parley, ult and AAs. Shaco has his Q, his Shiv, and his Clone. Zyra has her snare, her mini explosion thingie and her ult. Teemo has his blind and his AA's. Malz has his Silence, his DoT, and his Ult. Syndra only has her orbs.