Cosmo is very centrally located and has ideal walk through traffic. You may walk into it directly from sidewalk Strip level at either end, and also from the walkway into the second floor.
Across the street the Aladdin / Planet Hollywood messed up when they required all those steps up from the sidewalk to enter.
Even Caesars requires some effort to walk in from the sidewalk. Aria too, you have to be walking those bridge walkways to get to it easily, and then you enter not directly but via Crystals. Unless you take the tram to it from Bellagio.
No matter how easy it is to walk into Resorts World or Fontainebleau (which F. isn't directly on the sidewalk with an entrance right there like RWorld is kitty corner from Encore) they are just too far north to get the serious foot traffic. You have that section of cheap Mexican restaurants and Walgreen's before you reach Fontainebleau walking from Encore. People may lose interest and turn around before they even reach F.
Of course when Mandalay Bay was built it was predicted to fail for being too far south, but it did fine - but it still doesn't get a ton of foot traffic other than people who might be walking around Mandalay/Luxor/Excalibur or taking the tram between them.
One thing is that Fontainebleau does have a pretty big casino compared to Cosmo - 150,000 sq. ft. (Cosmo 100,000 sq. ft., Wynn 188,786 sq. ft., Encore 200,000 sq. ft., Venetian 120,000. sq. ft, Palazzo 105,000 sq. ft., Caesars 124,500 sq. ft, Bellagio 156,000 sq. ft., Aria 150,000 sq. ft.).
What is funny is that Fontainebleau doesn't like to say that they are across from Circus Circus. Oh no, they are across from Resorts World (even though - actually - they are across from maybe the last bit of Resorts World's lot (if you stretch the imagination), more across from - CC's structure, andTravelodge).