Allah has not sent messengers only to Arabia, Allah has said in the Qur'an that he has sent a messenger every people:
وَلَقَدۡ بَعَثۡنَا فِى ڪُلِّ أُمَّةٍ۬ رَّسُولاً أَنِ ٱعۡبُدُواْ
ٱللَّهَ وَٱجۡتَنِبُواْ ٱلطَّـٰغُوتَۖ فَمِنۡهُم مَّنۡ هَدَى ٱللَّهُ
وَمِنۡهُم مَّنۡ حَقَّتۡ عَلَيۡهِ ٱلضَّلَـٰلَةُۚ فَسِيرُواْ فِى
ٱلۡأَرۡضِ فَٱنظُرُواْ كَيۡفَ كَانَ عَـٰقِبَةُ ٱلۡمُكَذِّبِينَ (٣٦)
For We assuredly sent amongst every People a messenger, (with the
Command) "Serve Allah, and eschew Evil": of the people were some whom
Allah guided, and some on whom Error became inevitably (established).
So travel through the earth, and see what was the end of those who
denied (the Truth). (36)
Do not assume that messengers were only sent to Arabia for as the Ayah above says, a messenger was sent to every people and nation.
وَرُسُلاً۬ قَدۡ قَصَصۡنَـٰهُمۡ عَلَيۡكَ مِن قَبۡلُ وَرُسُلاً۬ لَّمۡ
نَقۡصُصۡهُمۡ عَلَيۡكَۚ وَكَلَّمَ ٱللَّهُ مُوسَىٰ تَڪۡلِيمً۬ا (١٦٤)
Of some messengers We have already told thee the story; of others We
have not;― and to Moses Allah spoke direct.― (164)