Other way around actually. Indica strains grow faster and are easier to grow. Most street cannabis strains are hybrid, where they are trying to get the high THC of sativa but with the favorable growing characteristics of indica.
I think the idea more generally (can't remember where I heard this, think it was one of the many documentaries focussing on dispensaries) was just that those growing 'street' weed aim for high THC and don't care all that much about CBD, while those growing it for legitimate medicinal purposes aim for high CBD with as little THC as possible.
People growing weed illegally for commercial purposes usually care more about weight and grow time than anything else. Of the actual breeders and geneticists who create the different strains, there are a variety of different factors that are optimized for. E.g. check out the webpage for apothecary genetics:
http://apothecary-genetics.spruz.com/
Most of the good breeders are in the Netherlands, but they are probably the top ones in the US.
"those growing it for legitimate medicinal purposes aim for high CBD with as little THC as possible."
That's definitely not true. The vast majority of medical marijuana is high THC. While many dispensaries are starting to carry high CBD weed and extracts, they'll usually only have one or at most two strains out of the 20+ they offer. In terms of actual medical use, different cannabinoids have different medical properties. So THC also has many medical uses, it's just not the only cannabinoid with medical uses.
Please do not generalize like this because it's actually harmful to active patients.