Because to the clown in the Oval Office, anything that doesn't support his Preferred Narrative

must be changed.  No matter the consequences.
A SOFREP source confirmed that after the shooting, Obama held a meeting in the Oval Office with his National Security Council, the attorney general, and the directors of the DHS, FBI, and NSA, in which a directive was given to downplay the terrorism angle. Such meetings are not held for mass shootings. Besides that, SIGINT aircraft like the ones the U.S. military flies over Africa (programs such as Creek Sand and Aztec Archer previously detailed on SOFREP) and Afghanistan don’t slurp up data from the skies over California unless the government is actively searching for other members of a terrorist cell. The same was done after the Boston bombings and the attempted Times Square bombing.
...
However, Obama’s most egregious, and dangerous, gaffe came when he stated that at the moment there is no intelligence indicating that the San Bernardino shooters received direction or funding from terrorist organizations overseas.  While Obama made this statement on television last night, the Department of Defense and Special Operations Command’s Threat Finance Agency has concluded that Malik and Farook most likely received funding from terrorists abroad.  With this in mind, the Threat Finance Agency has issued alerts to the appropriate banking institutions to flag financial transitions from specific banks accounts.
Admit it's terrorism, and he's afraid of pressure to actually DO something in Syria.  And Iraq.  He does not want to actually do anything, and wishes he could ignore the results of his actions.  And he desperately wants to ban guns- as many as possible, and restrict the rest as much as possible- before he leaves office, and if it's a terrorist attack instead of 'workplace violence'(like he called various other attacks for years), he can't really use it to push that.

Much like if he actually says and does anything about the Iranian violations of his treaty-by-another-name, it'll mean said treaty wasn't the genius move he desperately wants it to be seen as, so they play games and write stern letters.

Think about that: this bastard cares more about banning gun ownership by American citizens than he does about terrorists attacking us.  Amazing.