The OBC (Other Backward Classes) issue becoming a regional affair pursued at the state level rather than the central level can be attributed to various factors. However, the transition primarily occurred in the mid-fifties. Let's delve into the details:

1. The Mandal Commission:
- The Mandal Commission, officially known as the Second Backward Classes Commission, was established in 1979 to identify the socially and educationally backward classes in India.
- The commission submitted its report in 1980, recommending 27% reservation for OBCs in government jobs and educational institutions.
- This report sparked a nationwide debate on reservation policies, leading to the OBC issue gaining prominence.

2. Emergence of Regional Parties:
- With the rise of regional parties in the mid-fifties, there was a shift in the political landscape of India.
- These parties primarily focused on regional issues and advocated for the rights of their respective communities.
- The OBCs, being a significant vote bank in many states, became a key focus for these regional parties.

3. State Governments' Initiatives:
- State governments started taking measures to address the issues faced by OBCs in their respective regions.
- They introduced various policies and programs to provide reservations, scholarships, and other benefits to uplift the OBC communities.
- This shift towards regional initiatives was driven by the belief that state governments have a better understanding of the specific needs and demands of their OBC populations.

4. Regional Identity and Mobilization:
- OBC communities in different states started actively mobilizing themselves to demand their rights and recognition.
- They formed organizations, held protests, and engaged in political activism to voice their concerns.
- This regional mobilization further reinforced the OBC issue as a regional affair pursued at the state level.

In conclusion, the OBC issue became a regional affair pursued at the state level in the mid-fifties due to the emergence of regional parties, the recommendations of the Mandal Commission, initiatives taken by state governments, and the active mobilization of OBC communities at the regional level.