Because, since its creation in 1885, the Basingstoke constituency as been won by the Conservatives at every general election but one. And when they lost it, they managed to win it back the following year.
It is not accurate to say they have held the constituency since 1885 except for one year, because of the strange career of Andrew Hunter who was Basingstoke's MP between 1983 and 2005:
In 2002, he withdrew from the Conservative Party, in order to fight elections for the Northern Ireland Assembly as a candidate of the Democratic Unionist Party. He had family and Orange Order connections with Northern Ireland and opposed the Good Friday Agreement. The elections were held in November 2003, when he stood in Lagan Valley, and he failed to gain a seat, coming seventh in a six-seat constituency.
On 10 December 2004, he announced that he had joined the DUP Parliamentary Group in the House of Commons, the first Member of Parliament for a seat in Great Britain to represent a party based in Ireland since T.P. O'Connor represented Liverpool Scotland from 1885 to 1929.The one general election at which the Conservatives lost Basinstoke was that of 1923, when Reginald Fletcher won the seat for the Liberals.
And who was the Liberal agent who produced this unexpected victory?
In that great book Arlott in Conversation with Mike Brearley, John Arlott recalls:
"The doctor, I believe, recommended my mother to have another child and she said, 'Yes, Doctor, I would if only I could afford it,' and it really was like that. But she was a most capable woman. She was the local Liberal agent and did succeed. She was the only one who ever did, the only agent who ever got a Liberal in for Basingstoke.Reginald Fletcher was defeated in the 1924 general election. He later joined Labour and won Nuneaton in 1935. Raised to the peerage as Baron Winster in 1942, he briefly served as minster of civil aviation in Clement Attlee's government.