At that time you could have made similar software for JSP, ColdFusion or ASP.NET but hosting would have been much more expensive and a big hassle. With PHP it was feasible to offer a $5 a month hosting plan good enough for blogs and simple web application.
I wrote a lot of PHP because it was a great way to deliver products to customers, particularly ones that didn't have a lot of resources for "ops" but also a place where I could write open source software which might really get used and have an impact.