I haven't done a huge ton of web development, but I'll share what I know.
ASP.NET and everything else under the .NET umbrella (SharePoint, MVC, SQL Server, etc with C# as an exception) is locked into the Windows domain which means your server will have to be running Windows. Many servers however are usually under a Linux distro (CentOS or RedHat mostly) because of stability, security, and mostly no cost, which then means you're going to be using PHP most likely. That being said, there is a great demand in the enterprise space for being a .NET full stack developer and there is also value in your desk work space matching the server work space allowing for an easier development pipeline, especially for QA. Developing on Windows locally and uploading to a Linux server can have some headaches due to differentiating file structures so most developers usually have their local work space on the same OS as the server. There's nothing wrong with the ASP.NET approach, but you will be locked to certain platforms and companies specifically running as a "Microsoft Shop" where learning PHP and anything else under the LAMP umbrella (Linux, Apache, MySQL (also MongoDB ), and PHP (also Perl or Python)) will generally give you more opportunities and can run on any platform. A startup is more likely to go the LAMP route unlike an enterprise.
You could always learn everything, but we rarely have time for that so it just depends on what kind of opportunities you want.