Unfortunately, the answer is pretty boring: a lack of natural heirs. Perhaps due to strict monogamy alongside a fairly low upper class birthrate--not to mention assassinations and the like--natural born heirs reaching majority was relatively rare. In fact, Vespasian and Marcus Aurelius are the only emperors of the Early Imperial period who were succeeded by natural sons.

It is possible that this situation made the Roman court relatively harmonious compared to, say, the Chinese or Ottoman courts, where multiple wives and their families schemed for position, or later European courts, where there was enormous pressure to produce a male heir and the rules of succession were complex and highly disputable. But I am not sure how far I can push that.

Adult adoption was fairly common in ancient Rome, much more so than adoption of children. With the high infant mortality of the time, it didn't make as much sense to take in a child. Orphans were rarely adopted except by extended family.

Adult adoption, on the other hand, was often a way of naming an heir. It was not as simple as writing someone into your will; if you wanted to give your estate to someone, you had to make him your legal son.

A unique example is the populist politician Clodius. Originally from the patrician family of the Claudii, he had himself adopted by a plebeian family so that he could run for tribune instead of Senate, changing his name from Claudius to the more common sounding Clodius.

Something important has been missed: the roman emperors adoptions were not made to choose the best among the roman elite, has it has been depicted for a long time. Very recent research on brick making workshops around Rome (which were marked by the names of their owners) coupled with the genealogy provided in the Historia Augusta (wich contained some mistakes by the way) showed all the emperors of the IIth century from Hadrian to Marcus Aurelius were related, by marriage or because of a common grand parent.