Well, besides being the 2nd largest American company in the US and the store is available to everyone, that might say something about why everybody shops there.

Toys R US stores are about 90% kids clothes these days, Target some times has a selection and sometimes they don't, Hobby Lobby and Hobby Town mostly has plastic
cars vs. diecast (some of the metal cars they do have are kits that have to be assembled).

In short Walmart tends to have the best selection of diecast cars available across the US in general. We do have and HEB grocery chain in Texas with 300 stores and 56K employees and they also have great diecast selections but that only helps Texans.