fizzy - several reasons:
we both come from families of 2 boys and 1 girl, and the others were likely to follow traditional paths, so that by him and his brother keeping their family names, and my 2 brothers keepign theirs, it gave both families equal chances of the family name continuing to the next generation.
dh is from another country and his name is typical of that country. We were planning to live in a third country and then in UK, and no plans to live in his country, so we felt it was a way of continuing the link to his country.
His name is lovely and it sounds quite glamourous in UK!
HIs familiy are very very traditional conservative. While they wouldn't have flickered an eyelid at me keeping my name (see the bit in pp about my name in the bank) so we each had our own names, they would have been really upset if we had thrown out the surname all together and chosen another one, and they would have struggled I think if dh took my name. We would still have done it, but it was a matter of how much it mattered to us v. how much upset it woudl cause.
I loved the idea of chosing a brand new name for our new family, Dh would happily have taken on a new surname. It would have been my ideal solution, but I don't think I was brave enough. I didn't feel strongly enough about it to do it, and neither did he.
We had the added complication that any name we took would have been either an English name, or a name from his country, and so by default we would still have been giving preference to one side of the family or the other, so we couldn't find a neutral name.
No one reason took priority, but the cumulative effect was that we opted for his name.