2 reasons:

1. It deprives constituents of fully devoted representation
When officials take an oath of office, they are bound to fulfill that oath and dedicate their attention and activities on behalf of their constituency. By campaigning for any office other than for re-election, a politician’s time is spent away from and focused on something other than their present duties. It’s political adultery. If employers do not allow vital employees to work within the same field for another employer, because of inherent conflicts of interest, then why should politicians be exempt from this moral principle of responsibility?

2. It stops political springboarding
Given #1, constituencies may be seen as merely springboard positions to leverage into higher aspirations. For instance, it’s difficult to take Senator Barack Obama seriously as a fully dedicated Senator for the people of Illinois within his district with only 143 working days in office, and with many absent votes.

Legally, there is the question of fulfilling a fiduciary duty, when a politician wanders off to campaign for another office. However, if the office being campaigned for is reelection, then obviously, time spent exploring and discussing the relevant issues is in-line with the duties and responsibilities of the office, because the campaign process both informs the electorate and provides a means for feedback regarding their own concerns.