What is so difficult about mushroom cultivation?

What makes some mushrooms easy to cultivate is that they grow profusely, they aren't tree symbionts, where as sponge gilled culinary musrhooms are symbionts. Cheap mushrooms can easily be controlled with temperature and or light to fruit year round with minimal labour, they have cheap and easily obtainable substrate, and that are very appreciated for cooking and transportable. Also Argaricus doesn't even need light as a signal to trigger fruiting bodies and can be grown in complete darkness, and the harvest can be with robots not even manual, https://youtu.be/iBENPHl4mBA?list=PLw1YvKXHZymsoJbFGRI_OFSWZ5Wwg_fVD, which makes them particularly cheap.

Mycorrhizal mushrooms like boletes and many forest associated mushrooms differ from Argaricus and commonly commercially produced saprotrophic types (such as shiitake and button mushrooms). Saprotrophic mushrooms live and feed on dead organic matter, whereas mycorrhizal mushrooms grow in a close, symbiotic (mutually beneficial) relationship with the living roots of a tree.

Field and dead wood mushrooms generally have simpler substrate that can be transported in boxes in the dark, and almost all cultivated mushrooms are of saprotrophic.

Saprotrophic nutrition /sæprəˈtrɒfɪk, -proʊ-/ or lysotrophic nutrition is a process of chemoheterotrophic extracellular digestion involved in the processing of dead or decayed organic matter. It occurs in saprotrophs or heterotrophs, and is most often associated with fungi (for example Mucor) and soil bacteria.

That's slightly different to Saprophytes are more generally plants which live on dead or decomposing matter and which can also be more accurately be called myco-heterotrophs because they actually parasitize fungi, rather than dead organic matter directly.

They say that Argaricus was first discovered in 1650 on melons! but as any theory, there were probably precedents. Any farmer that discovered sizeable populatons of fungi near horse and cow middens could have easily have experimented with placing some compost in a basement.

Morelles and truffles are such expensive mushrooms that the man who learns how to grow big truffles will be payign himself up to a 1000 dollars a kilo for his efforts, it illustrates the difficulty of micorrhizal cultivation.

If you are interested in fungi there is a fascinating ted talk about them from Paul Stamets.

and https://en.wikipedia.org/wiki/Mycorrhiza