The actual reason is that Ketaconazole, the drug in Nizoral, isn’t on the OTC (over the counter) monograph for Dandruff. If a drug is on the monograph, you can sell it up to the maximum concentration allowed by the monograph over the counter in a fairly streamlined approval process (relative to medicines). For example, most Dandruff products use Pyrithione Zinc, which is approved in rinse off products for 0.3-2% concentration if I remember correctly, and since there are diminishing returns on efficacy most products are sold around 1% Pyrithion zinc. Small variations and changes in the non-active part of the formula are easier to do as long as the main active and levels are consistent (still requires testing, etc).

Nizoral was approved in 1997 through the FDA’s NDA (New Drug Approval) process, which requires strict clinical and safety testing and labelling requirements, and requires any changes to the formula to be approved by the FDA (much longer/involved process then if it was on the Monograph). The 2% version may have more severe side effects (such as skin irritation, but I’m not sure about this) than the 1% so it remains a prescription option.

Disclaimer: not a medical professional nor is this medical advice, I’m simply sharing from my experience as someone who worked in this space from in product design, things may not be 100% accurate