nivsy, the reason is simple... If the UK had agreed to all Schengen requirements (it's signed up to some, but not all, including the immigration control agreement), it would've been forced to apply Schengen requirements to everyone, including Commonwealth countries (like many African countries, the Asian subcontinent etc). The Commonwealth was still bigger (and still is) than the EEA at the time, so it was better politically to put the border control at all points to the continent, rather than include the UK as a whole in the Schengen area and leave the rest of the Commonwealth in the cold.
It was a political decision, something that does make sense looking back at the political climate at the time, but considering recent changes in the UK's stance on immigration, doesn't anymore.
S.