There are many levels of security screenings. The standard for a White House press day pass would only have checked public records, it would not have involved a detailed investigation. And on the tax issue, I've had IRS troubles, but it hasn't stopped me from getting a DOD clearance (twice).
A member of the WH Press Corps is going to get far more scrutiny than someone getting a DOD pass. Not only are you close to the President and administration officials on a regular basis, but you're much more important to the message the WH is trying to get out, and there are only a few people who are eligible to cover the WH in the first place because it presumes some professional background. Someone getting a day pass every day for a couple of years is also an unusual case.
This Gannon incident is obviously bad news, but it's such a stupid little thing that there's nowhere for it to go. Ooh, America cares about the WH press corps. Whatever.