Why HTML is not considered as a programming language ?
Because it merely a *markup language* without any logic. You can write anything you want and the browser engine will try its best to render the web page without any complain.
HTML means Hyper Text Markup Language
Because it doesnt have a functional purpose like programming languages generally have. E.g. you cant do maths, there is no expression logic, etc... This article goes into detail: https://ischool.syr.edu/infospace/2012/04/05/why-html-is-not-a-programming-language/