Why early rocket submarines used deck launchers

Could World War II submarine torpedo tubes be adapted to launch rockets at shore targets?

Not usually; early rocket submarines more commonly carried dedicated external rocket launchers.

Long World War II torpedo tubes were not a straightforward rocket-launch solution. Early rocket submarines instead carried dedicated launchers on the deck or stern. Tube length alone did not make a torpedo system compatible with rockets for shore bombardment. The weapon therefore needed its own external mounting.