my impression always was that either they ran out of time/resources or that they wanted something more modern and then chose against it later without coming up with a more appropriate vessel.
They’d also normally have multiple (smaller, often Coehorn) mortars instead of a huge mortar that would gradually break the ship.
Their name should also be Bomb Ship or Bomb Vessel as well - Monitor is such an awesome name with no relevence (at least Bomb Ship is self-explantory, lobbing bombs!).