A novel requires a certain kind of world-building and also a certain kind of closure, ultimately. Whereas with a short story you have this sense that there are hinges that the reader doesn't see.

A short story is one idea; a novel is a whole soup of them.

You can write a short story in two hours. Two hours a day, you have a novel in a year.

A short story is the shortest distance between two points; a novel is the scenic route.

What can you do if you are thirty and, turning the corner of your own street, you are overcome, suddenly, by a feeling of bliss - absolute bliss - as though you'd suddenly swallowed a bright piece of that late afternoon sun and it burned in your bosom, sending out a little shower of sparks into every particle into every finger and toe?...

A subplot is a distinguishing characteristic of the novel; the short story, for example, does not need subplots.