The biggest hurdle is that cancer cells move around within your body, alongside multiplying (even if you opt for chemotherapy: there's a probability that'll resist to the medicine or cause more harm than good). Also, Cancer cells aren't foreign invaders like the viruses, bacteria, etc that make humans sick, rather they are the person's own native cells that have "gone rogue" through mutation, making the process on finding a cure complicated.

If you really want to create a permanent universal cure: you will need to study and experiment literally every variation of cancer (it differs by individual too, as the treatment may work on one person but kill another: so the results are R&G to speak). This adds difficulity on creating a 100% antidote, as cancer is linked to a person's own cells (the genetic code varies based on person, it remains a complete mystery if the treatment will work).

The goal of cancer treatments is killing the cancer cells but (ideally) leave many healthy cells as unharmed (if possible), that's easier said than done. Most treatments affecting cancer cells are also going to readily affect a person's healthy cells too (that in itself causes more pain than comfort they would rather die than suffer). Current treatments only provide a working solution, but there's always a chance of resurgence post treatment.

Can Nanobots, mRNA or CRISPR guarantee a 100% cure with absolute zero resurgence on cancer: since how would you know if the patient's body will reject it immediately (even if you knew the person's blood type?). The true instigator of cancer is difficult to answer concretely apart from listing risk factors, since every human being in the world is different (from varying genetic code, health status, pre-existing conditions and etc).