Because it is the IG's job to filter out any complaint that has no basis in reality. Its the entire point of the system. It allows federal intelligence employees to rat out corruption while also stopping them from using it as a political hatchet tool. The IG already ruled the Trump complaint was credible and serious hence why it was moved forward. The ICWPA is also a lot different from other whistleblower laws in that it doesn't offer an legal protections to the person. So they don't have much to gain with coming forward, can still lose their job. The ICWPA also allows the person to go straight to congress if they feel the DNI is not respecting a credible complaint which is what happened here.
For the IG to approve a complaint it has to meet this criteria.
(A) A serious or flagrant problem, abuse, violation of law or Executive order, or deficiency relating to the funding, administration, or operations of an intelligence activity involving classified information, but does not include differences of opinions concerning public policy matters[;]
(B) A false statement to Congress, or a willful withholding from Congress, on an issue of material fact relating to the funding, administration, or operation of an intelligence activity[; or]
(C) An action, including a personnel action described in section 2302(a)(2)(A) of title 5, United States Code, constituting reprisal or threat of reprisal prohibited under section 7(c) in response to an employee’s reporting an urgent concern in accordance with this section.
If it was FAKE NEWS as Trump says then he wouldn't have Barr trying to block congress from seeing the complaint. Just like how if his tax returns were clean he wouldn't be suing everyone on the planet to block their release. If this was a Dem President trying to hide a whistleblower all you nutjobs would be flipping shit. But your Lord and Savior Trump can do whatever he likes.