Side-hinged doors have been on vehicles since the horse and carriage days. They are conventional because they are the easiest kind of door to engineer and manufacture, not necessarily because they are the most practical for facilitating ingress and egress in a vehicle.
The next easiest door to design and manufacture is probably the "suicide door" which made a return with Rolls Royce after 2000 and a more recent return in the Lincoln Continental, now that clamshell and blocking locks have made them safe. Getting into and out of a rear seat is easier with these doors than with forward-hinged doors, but they are difficult to reach when closing them without power assist. And, like conventional doors, they require a good bit of side space to open fully enough for an easy climb in and out.
The next easiest door to design and manufacture is probably the gullwing, which looks cool in ads but is a real annoyance to use in many settings.
Then comes the sliding rear door. Given how many years it took manufacturers to figure out a way to hide the track rails, they seem to be hard to design well and quite complicated to manufacture. However, they can allow for very wide openings (particularly useful for 3-row vehicles) and require the least side space outside the vehicle to open fully.
Finally comes the ultra-gimmicky falcon wing doors of Model X infamy, which are notoriously complex to build, reduce the torsional rigidity of the vehicle due to their interruption of roof cantrails, and apparently still have such alignment problems that Tesla resorted to a PPF film to deal with their rubbing the paint off the door sills. Their primary utility is for entertaining the neighborhood kids (of all ages).
I have been in and out of rear seats using all of the above door types except the gullwing. Sliding doors were by far the easiest to use in the widest range of places . . . as long as they were power operated, which almost all now are.
