The early ancestors of horses showed the presence of digits on their limbs, while modern horses have hooves. What are likely possible reasons for this change?

Natural Selection & Fitness

Explanation:

Horses have evolved to run as fast as possible because they are prey animals.

The hooves give advantages to the predecessors to the modern horse rather than the foot-bearing horses.

Thus when somebody inevitably becomes a snack, the hoof bearing horses lived longer. (Natural selection)

Because they've lived longer, they've had more babies. (Fitness)

Their babies are more likely to pass on the genes they've inherited from their successful parents, and the population of hooved-horses begins to exceed the population of footed-horses.

In this circumstance, all the footed-horses are gone. The population has evolved.

Hope this helps <3

PS: here's a source! https://www.amnh.org/exhibitions/horse/the-evolution-of-horses/on-your-toes#:~:text=Hooves%20and%20long%20legs%20help,many%20horses%20retained%20three%20toes.