That wasn't the reason. The F-5G/F-20 was always intended to be a FMS (Foreign Military Sales) like the F-5A/E and F-104.
The problem was, as a private venture, the most likely customer was Taiwan, ans both Carter and Reagan blocked that as part of the sucking up to Red China policy.
When it became the US Government F-X export fighter, it could only be sold through the State Department, and as warplane salesmen , they make great (well not anything really).
And there were always State Department approved exceptions to the policy: "the Pakis want F-16s? sure", "Israel wants US money to develop the Lavi? sure". "Turkey want F-16s? sure" "the Greeks asking equal dos? sure"
With State not pushing, indeed actually suberting, sales and the Reagan Administrations on-off policy on whether the F-X program should go ahead at all, Northrop finally pulled the plug.
The reason the F-20 didn't sell. Too much government.