Personal choice, maybe some regional influence. They're not so separate that you could classify them by country. I use the "deer" pronunciation, but if I slip into my Q'lander accent it's quite different (but still recognisable as "dear").

accents are applicable here with the pronunciation depending upon who is speaking, etc. This word is also a 'synonym' - 'Deer' is an animal, while 'Dear' is referred to as someone that is 'dear' to us(i.e.-friend, family member, etc). Examples - "My family is very dear to my heart". "My dear friend bought me lunch".

I've never heard it pronounced d-yar, and I'm British. It would be in old fashioned English but in modern English "deer" is universal.

''D-yar'' UK/AUS accent, "Deer" US accent? Correct me if I am wrong :)

Different accents!