Serialism essentially a way of composition to maximised dissonance. 12 tone technique ensures that all 12 notes are played in the chromatic scale before any notes is repeated to ensure that people do not unintentially repeat a note in the scale to give the piece an implied tonal centre.
If we leave what melodies people chose to write for chance there is a possibility that you will emphasise certain collection of notes more than others and as a result implicit tonality.
12 Tone technique removes that possibility and essentially maximised the possibility of not hearing a tonal centre in the piece of music (unless the composer deliberately manipulate the tone row to have triads aka Berg)
Aleatoric composition such as Cage - purely by chance, some colections of notes be emphasise over another and you have unintentional implied tonality. It sounds more ordered even if the composition was created purely by random.
Serialism is the equal and opposite of traditional tonal system. Both traditional tonality and serialism is highly organised music with series of rules. However serialism is highly ordered to minimised a sense of tonality and hence create music that is perceived to be disorganised.