Question

Why is the concept of orbit replaced by the concept of orbital?​

Answer 1

As the electrons behave like electromagnetic energy, we don't know exactly where an individual electron is. We know the probability of its being in a particular place. Schrodinger replaced Bohr's well-defined orbits with probability "clouds", called as "orbitals." 

Answer 2

since the bohr"s model put forward the concept of orbit,

that electrons are moved in fixed circular paths, called orbits.

                                         but

the existence of dual behaviour of matter put its wrong:-

de broglie"s hypothesis:-  every moving body posses some wave property with it. which are called de broglie waves.

, since electrons are also moving so they also poses wave property, and due to that wave property ,electrons are randomaly moves in any way instead of orbit. so this contradicts the orbit concept.

heisenberg"s uncertainity principle:- it it is impossible to determine the exact position and momentum of electron . which also contradicts orbit concept.

so , a new concept introduced called orbital , which considered a space around nucleus where probability of finding electron is maximum.

here , in the concept of orbit ,  it is fixed that electrons are found in orbit but the dual behaviour of matter concept contradicts it . so we move upto ,a space where probability of finding electron is maximum which introduced orbital.