Because the State Bank of Sikkim is the banker to the Government of Sikkim.
One of the functions of central banks is to be a banker to the Government (i.e. the government having a current account in the central bank). Thus, RBI is the banker to the Government of India, and the RBI Act (1949) allows it to be a banker to the state governments too if those respective governments desire so. All state governments agreed upon to have RBI as their banker, except Sikkim, as Sikkim already had it's own central bank -the State Bank of Sikkim - before it joined the Union of India in 1975 as a full-fledged state. Thus, the State Bank of Sikkim remains an autonomous body, not regulated by the Reserve Bank of India.
The Jammu and Kashmir Bank is Regulated by RBI.The bank is being inspected by RBI periodically .Though BR Act 1949 is not applicable to Jand K when the bank was to be included in Second Schedule of RBI Act 1934 , the conditions were applied to J and K Bank by agreement . They are maintaining CRR and SLR as stipulated by RBI.
The J and K bank is regulated by RBI ,Their branches in rest of the part of country was also licenced by RBI .
You can check up the list of scheduled banks in India on web site of RBI
Only bank remaining un regulated in India is State Bank of Sikkim ,
Jammu an kashmir