In this case there's a very simple reason. I'm one of the authors, for the newest version we did some restructuring and took advantage of the C# 6 Quality-of-Life improvements

For example the auto-property initializers introduced in C# 6/VS 2015 saved us about 100 LoC (on a 600 line class), and in addition made some properties truly immutable instead of privately mutable.

As for actually using the library, we provide .NET 4 and .NET 4.5 DLL's (only a 4.5 nuget package) so you can still use it in older VS versions, just not build it.

With VS Community Edition being free I don't see why developers providing open source libraries should restrict themselves to older versions.

It could be written with C# 6 and Visual Studio 2015 is (currently) the only version of Visual Studio to support C# 6.

You can see C# 6 features like the nameof operator in use here [1].

[1] https://github.com/spreadsheetlab/XLParser/search?utf8=%E2%9...