They can't feature them all. If they did, you'd disappear in a sea of freebies.
There are three bases to cover when planning a free run.
1. Have a good quality product to give away. Well formatted, editing up to scratch and an eye catching cover. It has to stand out. Here, reviews play a huge part. Many sites ask for 4.2* average or higher, and a minimum number of reviews too. This is often the hardest part of getting visibility.
2. Plan ahead. Most of the bigger sites take submissions 4 weeks in advance. Go as early as you can to maximise your odds. Diarise who you're submitting to, when and what you're sending in.
3. Describe the free product well. This is one of the areas authors fall down. All the freebies sites ask for basic details - Name/ Email/ Dates for promo (watch out for DD/MM or MM/DD!)/ Book title/ ASIN. Many of them give you extra opportunities to convince them - links to social media, descriptions, age rating, website links. Some of the more diligent sites check out your social media following when making a decision.
It's the description you need to work on carefully. Don't copy and paste the Amazon blurb (unless they ask you to). Something personal, a bit of history about you as an author, or some other unique facet will help you stand out. Think of it like a covering letter. You're applying for free advertising, not buying it (though many sites do have paid options). Get it right, and you'll get advertised on the big sites - BookBub, Pixel of Ink and eReader News Today can deliver five figure downloads. If you get those sites on board, Amazon will do the rest as you shoot up the download charts.