Is beta only be available for big investors?
The beta should be available to anyone but if you apply and get whitelisted.
Exactly, that's why it's called OPEN beta

I don't think big investors want to bother with open beta testing XD they will leave that to others.
I agree, earning is what they are interested in

Well they might invest even more if they like the beta.
Hm, I'm not sure about that. I think the serious investors have made up their mind on account of the project idea and know that the beta can have it's flaws.
Exactly, beta polishing of product is not a thing that would make too much impact on serious investors.