Frankly speaking, I'm also wondering why most Asian based casino use this VIP system which is based on deposit for specific timeframe (a month, 2 month, etc).
Maybe this is the best system to be applied on Asia or maybe this is the most common used VIP system by Asian based casino.
Of course it is just wild guess or spefulation only as the only party that know the reason is the casino itself but I believe the reason must be based on many things so they choose such a system.
I myself not a fan of VIP system based on deposit because I prefer VIP system based on wagering because I'm small player only who wont be able to make big total deposit in short period of time.
I think if the deposit method to increase the VIP level will be easy for gamblers to get.
For example, they make a large deposit and a few days later they withdraw the deposit capital and repeat the same thing. Is that considered valid to push our VIP up.
I also don't really understand the deposit strategy to reach a higher VIP. Because I often play at stake, the bets that are counted reach a higher VIP level, of course that makes sense and also requires a lot of processes to reach a higher rank.