For what reasons would an animal species NOT cross a *horizontal* land bridge?

In my worldbuilding project, 2 continents located on roughly the same latitude are connected by a land bridge. Both sides of the bridge are surrounded by a temperate grassland, and the lack of seasons means the climate is steady year round, so that rules out winter as an option.

The sentient aliens in my world, which I've been calling Not Humans, (very original) evolved on Continent A, and eventually spread out to Continent B across the land bridge where they found a species that wasn't present on Continent A. (Comparable in function to a horse) I haven't decided what the terrain is like on the bridge, but I would like to keep both sides of the bridge as grasslands if possible.

Why would these animals exist on only one side of the land bridge rather than both?

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For what reasons would an animal species NOT cross a horizontal land bridge?

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