It's because of the time-released ferts. The problems with the time-released ferts are:
- It can burn the roots and over-feed young sprouts because it gives them ferts when they should not be getting any ferts at all.
- You cannot control the type of fert or dosage, you get whatever comes mixed in already and released based on a schedule you didn't set.
- You definitely aren't getting separate veg ferts for veg stage and flower ferts for flower stage
- HID lighting in particular can cause the pellets to dissolve faster than they were designed, so that you get too much fert even by Miracle-Gro's calendar
- Those pellets can take a long time to dissolve, so if you are having problems you can't flush to clean your soil
There definitely are some MJ growers who use Miracle Gro with no problems. But there are many more who do have the above problems, so why use a product with so many potential disadvantages that are hard to fix?