Any sound explanation for this??

Because digitalis actually SLOWS the heart rate. That is why it is given to people who suffer from tachycardia and other abnormally fast heart rhythms!!

Digoxin suppresses the sinus node, mainly due to increased vagal activity. So only sinus bradycardia can occur, not tachycardia

For the same reason AV nodal conduction is also slowed which causes ventricular rate to be slow in atrial fibrillation.

Mobitz type I l block is due to conduction problem below the bundle of His, in the bundle branches, where digoxin has no effect. Hence Mobitz type I l block cannot occur with digoxin toxicity.