Why did Harry adopt Dexter only?

He explains the reason to be, Brian is older and too broken after seeing his mom being cut into pieces and would need professional care to be normal again. read more

Mainly because Brian was older, and would be required by state laws to have had mental therapy after undergoing such trauma. Dexter on the other hand, was too young for any therapy, and would be sent to an orphanage. Sensing this, he adopted Dexter. read more

Dexter, meanwhile, begins to question Harry's "code", realizing that he is the only one in charge of his "Dark Passenger". It is also revealed in season four that Harry had slept with many of his informants, not just Laura. read more

Dexter only had a few memories of the incident, it had an impact on him too, in future but at that time Harry thought it was reasonable to adopt him due to his young age. Also she was having an affair with harry and when Laura Moser(Dexter's mother) was working as a CI for Harry in a case, he brought Dexter with him. read more