Why are nearly all horizontal drivers circuits transformer coupled?
One (probably secondary) reason is that this provides one of the isolation barriers between a line-connected HOT and flyback primary and the signal circuits of the TV. A more important rational is that a transformer is nice easy way of impedance matching the horizontal driver circuit (100s to 1000s of ohms) to the few ohm input impedance of the horizontal output transistor base which requires upwards of several amps for proper drive. A typical driver transformer may be in the 5-10:1 turns ratio representing 25-100:1 impedance ratio. A byproduct of all this is that it is unlikely for a faulty driver stage to kill the HOT. Unlikely but not impossible.
