In light of recent revelations of Russia's involvement in the US presidential election, Trump's failure to release his tax returns may contain information relevant to Russia's interest in aiding Trump's bid for the presidency.
Why is this important?
There may be inappropriate ties that would greatly compromise American and global welfare in the event of a Trump presidency. This compromise would affect the US in particular, the world in general. If inappropriate ties are proven, Trump must not be inaugurated as President of the United States.