Right general idea. It's because he refused jursidiction. The subpoena might have said "Obama," but it addressed the President of the United States, who embodies the Administrative Branch of the Federal Government.
That Federal Entity refused to acknowledge any legal obligation to reply to a State Court under those circumstances, and the State Court chose not to pursue the issue.
That's just it though.
Because the administrative law of the federal government was created by the common/natural law of the States, the federal entity should have no choice but to respond to ANY question put to it by the authority that created it.