He (or someone involved in something he was promoting) was willing to pay for it.
Step 1 of getting a star on the Walk of Fame is asking for it. Step 2 is being selected from those "nominated." Step 3 is giving them $25,000.
If you want to fill this gap you can submit
an application on behalf of McCartney (though you'll need to include a letter of consent from him with the application). You and he will need to agree who is paying the 25k. If McCartney doesn't care to get one, and no PR person has convinced him to do it anyway, he'll never get one.