.NET Core is an open source and cross platform version of .NET. Microsoft products, besides the great abilities that they have, were always expensive for usual users, especially end users of products that has been made by .NET technologies.
Most of the low-level customers prefer to use Linux as their OS and before .NET Core they would not like to use Microsoft technologies, despite the great abilities of them. But after .NET Core production, this problem is solved completely and we can satisfy our customers without considering their OS, etc.