A thought for why it may have been easier for you to regularly use your paper journal while not maintaining the digital journal: it may be a matter of visual cuing.
With the paper journal it sits somewhere that you'll see it regularly, probably along your route to bed. Seeing the journal may have been your mental cue to write in it, as opposed to simple routine or habit. You don't get the same kind of cue from a digital journal because even a set reminder is no different from the bevy of notifications your phone gives you throughout the day.
Something that could help you keep up with your digital journal is to start keeping a physical journal in the same place you kept your old one. Maybe put an NFC tag in it that just launches your digital journal when scanned/tapped. Then you'll still have that visual cue and habit reaction force to keep you journaling.