It looks like there has been some backstabbing in Team trump.
In a Trump administration already plagued by chaos and disorganization, Flynn’s disruptive and confrontational style only amplified the perception that things at the White House weren’t running so smoothly. “Flynn had seemed like a dead man walking for a long time,” my colleague Yochi Dreazen writes.
“In the weeks before the Russia scandal,” Dreazen explains, “a series of leaks suggested that Defense Secretary Jim Mattis, Secretary of State Rex Tillerson, and CIA Director Mike Pompeo had lost confidence in Flynn and were holding private meetings expressly designed to exclude him.”
[source : The frontrunner to be the next national security adviser is basically the anti-Flynn, Vox, http://www.vox.com/policy-and-politics/2017/2/14/14610022/national-security-adviser-flynn-replacement-harward, 14th February 2017]
And Mattis trusts Harward enough that he gave Harward the task of planning war on Iran (who Mattis detests)!!
Harward also would work well with Defense Secretary James Mattis. When Mattis was chief of Central Command, Harward was his deputy. Mattis trusted him enough to put him in charge of planning for war with Iran. Mattis has urged Harward to take the NSA job.
[source : Robert Harward plans a housecleaning of Trump’s National Security Council staff, Foreign Policy, http://foreignpolicy.com/2017/02/15/robert-harward-plans-a-housecleaning-of-trumps-national-security-council-staff/, 15th February 2017]