This might seem obvious, but most companies get more applicants than they have open jobs in this field. So being a buyer’s market, employers can be as picky as they want.
First, you need to fill out all the questions on the application saying exactly what they want to see. Second, the resume needs to be perfect, as in no errors, looking very sharp and with a list of accomplishments, like degrees, previous workplaces, awards, etc. Fourth, the cover letter needs to be genuine, custom written and with impeccable grammar, spelling and punctuation. Third, the portfolio needs to be drop-dead wonderful, oozing with talent, and overflowing with taste, insight and good judgment.
For those having all those ducks lined up in a nice, neat row, interviews are typically granted. For those who don’t, well, they don’t get invited.