Major reason is the talent base of lpl is dead. It's mainly cause of the government rules and also cause of lack of interest in china. So the decline was bound to happen tbh.
Another reason is there are too many orgs or team in the league so even if there are any good talents they are scattered and are contract jailed in slave type contracts. for example one of the most hyped talent in recent years milky way is stuck in a bankrupt org rng with 10 years contract. Still tho all the talents are like at least 20+ years old which should tell you how bad the state of talent base is there.
And finally another reason is Chinese fans only care about wins not about #2 or #3 or even if lpl is a better region overall in the event so investment and popularity is decreasing since lpl hasn't been able to win any event for like 2 years now.
These are the main reason lpl is kinda doomed or is dying. and final nail in the coffin might be worlds this year since it's in china. The league will still sustain itself for few years until the old pros retire ig.