No heavy weight candidates have put their names forward because they know they will lose badly.
The purpose of a stalking horse candidate such as Owen Smith is to test for cracks in Corbyns lead. If he wins by less that last time, they can say he is less popular and should go, even if he beats Smith by twice as many votes.
Corbyn stands for anti-privatisation, nationalisation, social care, and a broadly socialust agenda. Funnily enough, that is what Smith says as well, so he is not a real alternative.