The virus is known to be an RNA virus. RNA viruses commonly mutate more frequently than DNA viruses, which helps explain how the new disease arose; basically, the virus changed from one that does not infect humans to one that does. It's similar to the common cold and to influenza, which are also known to mutate frequently. Some viruses adapt and others do not; Polio and smallpox don't adapt much and are therefore not so hard to control. This virus might be much more adaptable, and seems to be rather easily spread. It also can survive for some time on surfaces like countertops, which enables it to spread. It also has a relatively long and variable latency period, so it can spread insidiously.
This one might be here to stay.

Trump did it!

Animal to human cross infections has happened before

You make it sound like you think it was manufactured in China, like most everything else! It wasn’t of course, because viruses are not manmade, but are agents that need living cells to replicate in animals and plants. They are clever little buggers and can mutate and evolve into new strains, that’s why we keep hearing of new ones. Each year the influenza virus needs a different vaccine because it has evolved into a different strain, and as @Matias has already pointed out, countless thousands of people die of the ‘flu annually, despite there being a vaccine available.

As @Matias says, good old-fashioned evolution by natural selection.