Applets implementations were terrible - the problem wasn't so much Java ( though early versions pre-jit were slow ) , but the interface between the browser and the applet.

Memory leaks abounded in particular.

Life cycle management was difficult as well.

Note the interface had to be implemented in each and every browser separately - compounding the problem - for applets to be viable it had to work on all the major browsers well.

Not blaming the people who worked on it - I suspect the origin design was put together in a rush, and the work under-resourced, and it required coordination of multiple parties.

Java's constant security nightmares and lack of integration into browsers was the bigger problem, IMHO.

Also you had to install runtime vs browser which is bundled every OS.

I think everyone instinctively knew that “once it is in the browser it cannot fail”, so it has been a schelling point for tech.