It takes time to convert people and often you may only get 5-10% of the guest players to deposit. If you’re offering free money(without a deposit), as some casinos are doing, you’ll find very few depositors.
I’ve sent tons of players to party poker over the years and i’ve picked up depositors from campaigns as far out as 2 years after they first downloaded the software.
Combine these two points and you may decide that the coversion rates you are seeing in the short term might be reasonable numbers.