First I think a quick clarification is in order, to the effect that the Russian Black Earth region (chernozem) is technically in what is called a "forest steppe" region, meaning that it is mixed grasslands and forest. This region is south of Moscow and includes such modern day Russian provinces as Voronezh, Kursk and Tambov. If you look at a map of, say, Kievan Rus' principalities in the 13th century, you can see that this region is near nomadic communities, but was a part of settled, agricultural Rus' territory.

But that's just a minor clarification, because the area that is "proper" steppe is also incredibly fertile for agriculture, and was definitely historically controlled by nomadic peoples. The part closest to European Russia is often called the Pontic-Caspian Steppe, but it's actually part of a much, much bigger steppe zone.

The steppe area in present-day Ukraine and southern Russia has a particular history, at least in modern times, that helps to explain why it was not turned into agricultural land until relatively recently. After the disintegration of the Mongol-based Golden Horde, this area largely came under the rule of the Crimean Khanate, based out of Tatar-inhabited Crimea, which was a client state of the Ottoman Empire. The area north of Crimea was a borderlands region called the "Wild Fields" - it was in effect a warzone that saw numerous repeated slave raids (famously or infamously including those conducted by Crimean Tatars for Ottoman slave markets, but it's worth pointing out that the slave trade did also go in the other direction too). The Russian state didn't establish a strong presence in the region until the late 18th century, and the Crimean Khanate was only finally conquered and annexed in 1783.

It was after that event that Russian and Ukrainian (national identity in this period is a long, complicated and acrimonious story) settlers began to move into this region, known as "Novorossiya" (or "New Russia") and develop agriculture.

A similar process occurred in modern-day South Russia as well, as Russian government-supported Cossack groups established stanitsas (settlements) and "lines" of defense against various Caucasian peoples (Circassians, Chechens, Dagestanis, etc). This process was relatively slow in the 18th century, and then picked up pace and intensity in the late 18th and early 19th centuries. Slavic agricultural settlement was part of the border defenses, but once the frontier moved further into the Caucasian highlands, larger scale agriculture became possible.

OK, so I mostly breezed through the Ukrainian and South Russian steppes, so I can talk about the bit I know more about, namely the Kazakh steppes, and can get into a little more detail as to why such agriculturally fertile land was used by nomads for centuries instead of farmers. Bear in mind I'm not a farmer or agricultural specialist, but will source my info.

The Kazakh steppe region possesses some of the most fertile soil in the world for wheat production, but...there are some major "buts" around using this area for agriculture.

First: the temperature. The steppes have an extreme continental climate system. Very hot summers and very cold winters. It's not unusual for cold snaps in the winters to reach -40C (-40 F), and for heatwaves in the summers to reach 40C (104 F). These are massive, massive fluctuations when compared to milder agricultural regions, and crops need to be hardy and fast-growing in order to take advantage of the growing season, such as it is. If you're sowing wheat in the region, often it needs to be sown mid-May to avoid late frosts, and that wheat crop needs to be ready to be harvested before early frosts set in in October. Annual weather changes can endanger a crop, and the cold soil makes crops susceptible to particular diseases such as root rot.

That's temperature. Now on to water. You can have the most fertile soil in the world, but if you don't have water to irrigate your crops, that soil won't do much (ask anyone farming in the American Southwest). In the case of the Kazakh steppes, they are a bit outside of the zone in Central Asia where agriculture developed complex irrigation techniques such as canals for large scale farming. As such, most of the agriculture in this region relies on rain, and rainfall is relatively meager - the US Department of Agriculture gives an estimate of some 20 inches of rain a year, compared to 30 inches for Kansas. As if that wasn't enough, rainfall in this region (as well as the Russian Black Earth region) tends to come from the Baltic Sea and can be highly variable, and often come at precisely the wrong times of year - not in the spring, but in the summer when heavy rainfall can actually damage young crops. So it's a little bit of playing the lottery to see whether rainfall will give you a bumper crop, or wipe you out for the year (which is one reason why the agricultural communities especially in the Volga region often historically saw periodic famines). If you're farming in the area, you're probably going to see a serious drought in two out of five growing seasons.

Finally, let's check in on that soil. The Chernozem is very fertile, as is the so-called "brown soil" (kastanozem). However, these fertile soils are not evenly distributed across the steppe zone (I believe they tend to be located mostly in valleys but don't hold me to that). Fertile soil zones are mixed together with highly agriculturally-unproductive soils such as the salty solonchak soils. So it's not enough to just assume you can farm anywhere on the steppe - you need to know exactly where you can farm, because otherwise your agricultural venture will face disaster (this was part of the problem with attempting to expand agriculture in Kazakhstan in the Virgin Lands Campaign in the 1950s and 1960s - many of the regions converted to agriculture were marginally-suited to farming at best).

A final note should be made about the nomadic populations on the steppe. The idea that nomadic peoples just, you know, wandered around with herds of livestock is not really accurate - nomadic communities had set areas of pasturage that they moved to seasonally in a cyclical pattern. While their agriculture was limited, it would be a mistake to assume it was nonexistent. Scythian grain from the Ukraine, for example, was a major food source for classical Athens. In the 19th and 20th centuries, partially at the encouragement of the Russian government, and partly because of internal reasons, Kazakh pastoralists also engaged in limited agriculture in their seasonal pasturages, growing winter wheat and fodder for livestock.

Sources:

King, Charles. Ghost of Freedom: A History of the Caucasus

Olcott, Martha Brill. The Kazakhs

Pipes, Richard. Russia Under the Old Regime (mostly the geographic introduction)

US Department of Agriculture. "Kazakhstan Agricultural Overview" (which can be read here)