Why are there so few Roman emperor directly related to each other?

The reason is twofold: first, they were related. Second, emperors weren't Monarchs and the republic never developed a fully understandable succession system.

I say "they were related" because Roman understanding of family was different than ours; political, acquired members of a family were as family as carnal family. In that way, having a set of able (or contingently politically strong) was advantageous both for the state and for the survival of the "dinasty". Emperor got to be Emperors for a lot of contigent reasons, but even if you could become the emperor by sheer luck, terror or brutality, staying an Emperor needed the acquiescence of at least an important number of the constituent groups of the republic, so legitimacy was key. Dinasty, as in "belonging to the ruling family" was a source of legitimacy, but only a preentive one and not the only one, or the most effective. Even if Emperors tried to stablish dinastic principles they never worked, at least up untill the post 4ty crusade XIII century period.

The longest ruling dinasty would be the Macedonian Dinasty, and it's a good example of how "dinasties" work: the founder was a nobody who ascended to the top of the state, and the lineage was precariously, sometimes even artificially, maintained by political alliances, incorporations and military strongmen ruling "in the name of the eirs". Even the greatest of it's members (and the indirect responsible for its ending), Basil II, spent the first part of his reign ruling in name only; he had to destroy the political team put by his grandfather, Constantine, whom himself had suffered strongmen "protecting him and ruling in his name" untill all of them died and he had to actually govern. For some reasons Roman people of that time seem to have loved the dinasty, as the people took the streets every time its survival was in danger, as exemplified in the story of Zoe and her brother, saved by the populace from a coup (by Zoe's husband), and then forced to rule together despiste Zoe didn't want Theodora at her side. Theodora had been outwillingly dragged out of a convent by said population, who ended up imposing its political agenda, as Roman people tend to do when they got serious. But again, dinasty was a contingent tool people had to get to, or remain in, power, but not a definitive one and, in different eras, kind of irrelevant really, while in others it was more of a show/way of structuring power among the elites than actual dinasties. It's worth nothing there was no idea of blood aristocracy, and "aristocratic families" got aristocratic by getting close to power, not by blood. Power was dependent on the State, too, not so much on money itself, as the main avenues to power were thru the state: military prestige was aquired thru the army, a state institution; state salaries and revenues for state services (as tax collection) were main sources of income, eclesiastical power (a member of the clergy couldn't be Emperors, but they could make and unmake them in specific circumstances, and they were always political players) was aquired thru Church, which was a state institution, and even commerce (not a prestigious thing to do in Rome, you won't see any rich by commerce emperor and a lot of rich by landowning Emperors) was administered by state oficials. As such, the state (the republic) had so many actors and factors playing that it was impossible for a single family if not to grab, then to maintain an "aristocratic", aka "with enough legitimacy to claim power", for more than 3 or 4 generations. That happened, too, to Emperors. No unpopular Emperor could rule for long, so it's only obvious that a dinastic system, if present, would be flexible enough to be considered dinasty by name only.