One reason would be that "Senator" alone wouldn't be too useful on a global tree. By using the display name field, we can specify the country.
Also, I think we determined that "Senator" is one of those forms of address that's a position, not a title, but I might be wrong about that. I know it came up in the (many) discussions about the field when it was first introduced.
Personally, I like the brevity and clarity.
Kevin Lawrence Hanit: I don't want to stay off-topic, but why lock it? Any user can update her living status rather than wait for you.
The proper term of address is “the Honorable” but that’s not descriptive of position.
It is necessary to have US, not only to disambiguate from other countries, but from state senators.
Most titles are in the suffix position with the honorific in the prefix position, i.e.,
Sir John Smith, 1st Baron Smith