HTML5 was new and not widely supported, the web was WAY more fragmented back then, to put things in perspective, Internet Explorer still had the largest market share, by far. The only thing that could provide the user with a rich interactive experience was Flash, it was also ubiquitous.

Flash was the biggest threat to Apple's App Store; this wasn't a conspiracy, it was evident back then but I can see why it is not evident to you in 2025. Jobs open letter was just a formal declaration of war.