Question

why the neutron was discovered so late as compare to electron and proton

Answer 1

because electron and proton have charges so this was easy to find whereas neutron is neutral and don't have any charge so it was discovered later

Answer 2

Because neutron had the charge zero and no on cared about it as it was not required in anything but proton and neutron represents specific charges and are in reactions...

(: Hope it helps :)