Let's take a step back and look at two interrelated concepts: the two kinds of motion expressed by Russian verbs of motion and the two aspects of verbs.
Unprefixed verbs of motion are all imperfective. That means that they refer to motion which was, is, or will be taking place at the time named. For example:
Я иду в магазин.
I am on my way to the store.
Я шёл в магазин.
I was on my way to the store.
Я буду идти в магазин.
I will be walking to the store.
Verbs of motion come in two forms:
Unidirectional (which you call concrete):
Я шёл в магазин.
I was on my way to the store.
Я шёл с рынка.
I was walking from the marketplace.
This implies purposeful motion leading to a predictable change in the traveler's location. It implies physical progress.
Multidirectional (which you call abstract):
Я ходил по магазинам.
I visited the stores.
Since ходить describes motion in various unspecified directions, it is difficult to say where it leaves the traveler. Thus, it does not imply progress.
How is this related to the question of which which motion verbs become perfective when prefixed? Remember that the perfective aspect of a verb refers to an action or event which the narrator wishes to portray as completed and as advancing his narrative (or argument).
Let us explain by starting with an imperfective example:
Мы пили молоко и пели песни.
We were drinking milk and singing songs.
While this may have been a fun way to spend an evening (or not), it does not take the plot in any obvious direction. This statement does not prepare the listener for any new developement of the plot. Compare this to the following perfective statement:
Я пришёл домой и нашёл письмо от адвоката.
I got home and found a letter from a lawyer.
Here we have clear narrative progression. The speaker's unidirectional motion toward his home was continued until he arrived. Because he arrived he was able to find the letter. Here we see that the verb прийти expresses an idea (arrival) which advances narratives. The ability to express such an idea (rather than some morphological feature) is what makes it a perfective verb.
Note the distinction between making progress (which идти does) and advancing the narrative (which прийти does). A character's physical progress on his journey does not advance the narrative to the next step until it fulfils some defined goal. The goal is specified by adding a prefix such as при-, у-, от-, об-, с-, на-, до- etc. These prefixes are prepositions doing double duty. They specify a relationship to physical objects (such as persons or landmarks) which the motion is to achieve. Reaching such a defined point turns the motion into a perfected fact able to advance the narrative to the next step.
Perfectivity of a motion verb requires both these elements: the motion must make progress, and it must reach or pass a goal post. By doing so it produces an effect which advances the narrative to the next step. Only prefixed unidirectional motion verbs provide both elements.
Prefixed multidirectional verbs of motion define the goal, but because they describe aimless, circular, or repeated motion, they do not move the character to a specific location for the next step of the narrative. While "Анна пришла в школу." places Anna at the school for the next stage of the action, "Анна ходила в школу." does not. Now she could be anywhere, quite possibly at home. Thus, prefixed multidirectional verbs of motion are seldom perfective.
