I agree with
@bumbershoot that the perception of shampoo varies from hair type to hair type. For my hair the H2O shampoo and conditioner are horrible. I look like a mop when I use it. My partner however loves it and his hair is wonderful and soft when using it.
I also think it is so popular because it has a very distinctive scent that reminds people of how much they enjoyed the cruise. It is also a true unisex scent, not too sweet and not too masculin, just very neutral. I think that this helps with the general appeal.
Personally I love the body lotion (never had the body butter, started cruising just after this was changed) as it is very light and not greasy at all. I find that true of all the other H2O body lotions I have tried at Disney (I still mourn the aquatic series they used to have at the WDW resorts). To me that is a sign of quality, but for someone else who prefers a very heavy body lotion, they might not like this.