Because at the time I started using it, it was still Mozilla and the other alternative was Internet Explorer, the browser from Microsoft - the Evil Empire of that time - which was riddled with security holes (the whole ActiveX stuff, especially, was a complete total mess).
Later Chrome became more fashionable but to me it was already obvious that Google's "Do No Evil" slogal was complete total bollocks (plus it came out in the Snowden revelations how Google was used for civil society surveillance, plus by then they had become mainly an Ad Company with a search engine, hence anti-privacy) and I wasn't about to trust what already back then looked like the up and coming New Evil Empire with access to my computer and browsing habits.
Mind you, I did use Chrome on my Android devices, but that was because I expected the OS itself to be rigged like crazy for privacy intrusion and worse so avoiding Chrome there did very little to reduce my privacy exposure in there, though eventually I moved to Firefox there too.