When formulating a strike plan, the planners assess the likelihood of success. This strike has been "war-gamed" numerous times. The worst scenario is one where an attack occurs, but the outcome is unsuccessful. Now, open hostilities (war) exists between countries, counter-attacks are certain, and the nuclear sites remain functioning. Additionally, all the leverage of a possible destructive attack has been lost. (This is similar to Carter's failed hostage rescue attempt, minus open hostilities with weapons as it was not an attack, per se.)

Now the US is in a bad spot. Do you attack Fordow and the other sites again now using tactical nuclear weapons? The use of nukes is a HUGE escalation, and one the world will condemn regardless of motives and justification. You've also invited a nuclear counter-attack upon the US homeland.

This is a very difficult strike using only conventional weapons due to the depth of these nuclear facilities. This is precisely why they are located deep underground.