Leftists is a large and ill defined grouping. Leftists include liberals, marxists and many anarchists. "The individual" is also a loaded and poorly defined term. To answer this question in my own way I will assume that by individual you mean something consistent with the thoughts, passions and writings of egoists and nihilists.
The are a number of reasons why a focus on the individual could be decried as bourgeois by people who identify as leftist. The extent to which a notion of humanism, a concern for the whole human community or the central importance of humans in the world, inflects the political positions of these groups it signals the degree to which the abstraction "whole human community" or "humanity" is the sacred cow of their politics. To focus on the individual or oneself or the unique is a turn towards decadence and a failure to properly value the human community/humanity. It is putting one's own interests ahead of the abstract groupings (the people, the party, the masses, humanity, etc.).
To the extent that leftist political positions harmonize with enlightenment myths of progress (to the extent that they fail to challenge a progressive unilinear historical narrative) they will be judging thoughts, actions, intentions, and peoples effects in and on the world from the standard and position of this thing called progress (however they define that abstract notion). To betray a commitment to the unified story that progress implies (in reality, demands) is to sacrifice the needs of all or the needs of the many for one's own selfish interest. If we turn the rhetoric of the last sentence upside-down in the interest of demystifying it we get the statement, "people/individuals must sacrifice themselves and their interests for the greater good, for humanity, or for progress". These three abstractions are foundational to the stories and moralities that dominate us in this world.
If we are talking about revolutionary leftists the concern is slightly more nuanced. Revolutionary leftists are concerned with finding the form of social organization that is capable of overturning the world as we know and hate it. In some theoretical and activist circles this is referred to as the revolutionary subject. This is the collectivity that, in theory, will be capable of applying enough force in a concerted manner to destroy the order and systems that dominate us. From this revolutionary perspective there is no set of individual actions capable of coalescing into a force capable of smashing the state or destroying capitalism and focusing on the individual is either bourgeois or even counter-revolutionary. In this scheme, the revolutionary subject or party is the abstraction or political form that allows for the utopian revolutionary dream. In this case the revolutionary body is the force that determines good and bad and our morality and revolution is the progressive narrative that captures and stultifies our unique passions or our singular drives to experiment and exult in life in ways not subjugated to masses and herds, party or platforms, abstractions or causes.