The reason is that liquid ones are idiot proof and ready to go. People can mess up easier with the powders, make a mix that is way too strong, and kill their plants with nute burn.

When i was getting Big Bud from Advanced Nutes, i was told to try the liquid first bc if you don't mix in the powder fully, it can kill your plants burning the roots.

So if you got some brains and dont smoke before mixing your nutes - your fine using the powders