I'd say it's because HN is a bit of a "hipster" crowd and there are more startups, with fresh stacks that want to use new exciting technologies, than corporations with a "solid" base.
that explains Python which I suppose includes Django, and Vue.js
Vue is backend agnostic. You can use it with anything.
I think that the parent post could be better written as:
> Python (which I suppose includes Django), and Vue.js
it's too easy to read it as:
> Python (which I suppose includes Django, and Vue.js)
I wouldn't regard PHP as solid or corporate. Rather it's usually a sign of a business which started out as a website made by less-technical people. (Which can be the best kind to join as a technical person, as it means your skills are likely to be complementary to what they already have).
It demonstrates that their product is old and web-based, yes, but not that it was put together by less-technical people. In the mid-2000s, PHP held like 80+% of the server-side programming language market, ASP was in the mid 10s, leaving every other language to fight for the remaining 5% or so.
I don't see how it demonstrates that it's old. I still constantly talk with very new startups that are using PHP.