The easy answer is precision.
Electroconvulsive therapy is the mental / medical equivilent of pounding on the TV to "fix" it, and is a last ditch treatment used when the simptoms cannot be treated any other way.
It, as the name implies, creates a seizure in the paitent by applying voltages across rather wide areas of the brain. Originally uncontrolled voltages (because it came through skin, bone, etc) into, nearly random brain tissues (because it was applied in ignorance, through multiple barriers, with no guarentee as to where the potentially (and often actually) damaging current went. Just as pounding on older electronics minutely shifted ALL components, sometimes cleaning a little bit of oxidation, or closing a curcit board crack, allowing current to flow where it had been blocked.
Today, the seisures are usually limited to purely electrical storms in the brain, and usually do not present themselves as convulsions, or uncontrolled muscle spasms.
I assume that the procedure has improved over the years, but I do know for a fact that ECT, to this very day, often destroys short and mid-term memory. Who knows what subtler damage it can cause?
On the other hand, Stimulating one, or a small group of cells, is nothing like ECT.
In fact brain surgeons will stimulate small pockets of brain cells before and during surgery (invoking memories, sights, sounds, tastes, sensations and smells) to minimise the chance of damaging or removing cells unrelated to the surgery. Something like that could never be done with ECT.