Why did people think it was unconstitutional to have a national bank?
Andrew Jackson thought that the BUS (bank of the united states) was biased against westerners.
None. The US Supreme Court declared Congress had the constitutional authority to establish a national bank to handle the United States financial transactions under the Necessary and Proper Clause in McCulloch v. Maryland, 17 U.S. 316 (1819). In other words, the Supreme Court declared the national bank constitutional, not unconstitutional.
They believed it was unconstitutional because it was not stated in the constitution that one of the powers of the federal government was to create a national bank. Hamilton however said that it was needed to enforce taxes and that because of the "necessary and proper" clause, they should create a national bank, so they did.