I think that it is the lack of knowledge of the people who do not speak the Arabic language and for those who speak it naturally, lack of education.
When addressing 1 person male, it should be said:
Alsalam Alika
السلام عليكَ
While addressing a 1 person female, it should be said:
Asalam Aliki
السلام عليكِ
For example, take the following Hadith
قَالَ أَبُو عَبْدِ اللهِ وَقَالَتْ قَيْلَةُ قَالَ رَجُلٌ السَّلاَمُ عَلَيْكَ يَا رَسُولَ اللهِ، قَالَ وَعَلَيْكَ السَّلاَمُ وَرَحْمَةُ اللهِ
Qayla reported that a mans aid, "Peace be upon you, may Allah bless him and grant him peace." He said, "And peace be upon you and the mercy of Allah."
The same is true when viewing many other hadiths of the same nature. It was never reported from the Sunnah or general Arabic culture about 1 person meeting another 1 person and giving him/her the salutation for the many, just doesn't make sense.
