Under the terms of the deal, Boeing will build a new version of its F-15, an older fighter jet developed in the 1960s, with new automated flight controls and electronic warfare capabilities. The newer model, known as the F-15EX, will be designed to carry heavier weaponry than smaller, stealthier jets like the F-35.

The Air Force is looking for a bomb/missile truck. They already have extensive experience with the F-15, and retraining(pilots/maintainers) and retooling to use the smaller f-18 would most likely cost as much or more as this deal. Also given military structure you would have to spin up a whole new command and maintenance Wing for the new type. Where the f-15 will just get added to an existing f-15e wings.