It is important when analyzing kurgan remains that these represent societal elites, and thus aren’t necessarily representative of the larger population, as is the case with R1a lineges being overrepresented among Brahmins in India.
That being said, the sudden rise in frequency of L51 among Corded Ware males might be more than a random event, as these lineages could have been more widespread among Yamnaya commoners.
Meanwhile, the current distribution of R1b-Z2103 seems to show a clear link with Balkanic IE languages, with clear traces of Greek and Phrygian expansions into Italy and Asia, respectively.
My hypothesis is that Yamnaya chieftains were attracted by the wealth of the Cucuteni Trypillia culture, which seems to have been the most developed culture at the time (though a lack of Neolithic excavations outside Europe might have something to do with that).
Meanwhile, the less developed territories to the West would have been the crumbs left to the lower strata of society (which could have been lower nobility as well).
As for the reasons why lower class Yamnaya decided not to follow through, a plethora of hypothesis could be made. For instance, there might have been warrior codes which forbid them from attacking their brethren, especially if of noble bloodline.
Moreover, such expansion might have been more peaceful than once thought, and could have been driven mostly by marriage alliances where Yamna chieftains secured links to the wealthiest families.