Because it is the same as the cold virus, which no one have ever developed a vaccine for
It’s in the same family “coronavirus” as some of the cold viruses. But there are more than 200 viruses that cause the common cold and most of those are not coronavirus.
That’s why there is no vaccine for the common cold, you would need 200 vaccines.
This particular coronavirus, “SARS-CoV-2”, which causes the illness named COVID-19 is much worse than a common cold or the flu.
There are hundreds of cold virus strains, so a vaccine would generally be useless.