Microgreens need water, and your soil should provide it in a balanced way. Bad soil could lose moisture too quickly, lowering your germination rate. Soil could also hold too much water, giving way to diseases, which kill your seedlings (and lower your germination rate). Soil should also be sterile and disease-free.
Sounds complex, but it actually isn't that hard. Just find a good quality seed starting mix. Most worked well for me with the exception of Miracle-Gro Seed Starting Mix. Here are some of my favorites:
Jiffy Organic Seed Starting Mix - This is in stock at both Lowe’s and Home Depot for me. At under $5 a bag, it is relatively cheap. The quality is also good, with moisture retention staying high while being light and fluffy. It can have some larger pieces in it that I have to pick out, but not a big problem for me.
Burpee Organic Seed Starting Mix - I also use this from time-to-time. It isn’t as cheap at $6 per bag but gains higher quality.
Coconut coir. This is a medium made from coconut fiber, but the maybe the most valuable thing about it is that it can be compacted down to 1/5th the final size. This means that it is one of the few soils that is easily shipped. One block the size of a brick can give you a 4 quarts of soil.
Hydroponics and Grow Mats. These are very viable options for microgreens, but I don't have much experience with them. I prefer soil, but many growers use hydroponics and grow mats with success.