The simon commission of 1928 was boycotted by the congress because:-
It definitely negated the claim of the Indian people to determine their own constitution
Though the first statement also seems to be correct but we cannot say it was the main reason why congress boycotted the commission.
Simon commission:- All white, seven member Indian statutory commission appointed in 1927. It was to recommend to the government whether India was ready for further constitutional reforms and on what lines. The commission landed in Bombay on February 3, 1928. Countrywide hartals were organized.
The Indian response against the commission was immediate and nearly unanimous. What angered Indians most was the exclusion of Indian (And Not only the congress) from the commission and the basic notion behind the exclusion that foreigners would discuss and decide upon India’s fitness for self – government. This was against principle of self- determination.