Today, I woke up randomly wondering If it is just me or, it is has been very rare to find cactus farms that are made out of red sand. Thinking about it, I decided to look around and I was right. Apart from /kit red sand costs the same as normal sand, so the question is, is why do people use normal sand on skyblock specifically instead of red sand??!?!
i believe it's because of something you already mentioned. regular sand is in kits and more "accessable" than red sand. while a lot of players do buy sand from /shop and oher players through trades or ah, it makes more sense to be uniform and not switch up the sands. a red sand cactus farm seems impractical since you could essentially be losing money especially since cactus dont make a whole lot of money by themselves compared to netherwarts and melons- players would have to sell their regular sand from kits (usually for less than 15% of what /shop is) and then buy red sand from /shop with or without the 15% perk it just seems like a time waster and money waster. as a concluding statement to your question, i'd say it's because it's impractical to use red sand for farming purposes when you already get regular sand from kits i'm sure there are more reasons to it, but these are the reasons i came up with Have a great day!
I guess red sand could be used for people who don't have ranks? Or, if you do /kit that day and run out of sand? Although, I'm not sure that cactus can be placed on red sand.