There's no point in asking "why" when it comes to language. It is completely pointless, and there is no answer other than "just because".
There is no intrinsic reason whatsoever for us to say "there's no point" and not "it is no point", or "it's raining" and not "there's raining" — no reason other than that we have to agree on saying something, and that's what we happen to have agreed upon, and so we're good.
Yes, you can try and come up with a "rule", but that "rule" would be merely descriptive. It would be an attempt at justification in hindsight. It would merely describe what we're doing anyway. Describe but not explain. And even at describing it would typically perform rather poorly, introducing exceptions, or ignoring them. And it would completely ignore the fact that languages constantly evolve, and nothing at all stops people from agreeing on saying "there's raining" instead, gradually rendering "it's raining" ungrammatical nonsense.
So the only honest answer to your question is, you must use "there's no point" because that's what every native speaker at this point in time would use, and you must not use "it's no point" because that's what no native speaker at this point in time would produce.
Keep in mind that no native speaker acquires the language by learning rules; every native speaker acquires the language by simply repeating after other native speakers. I suggest you do likewise.