Why is Lord Alfred Tennyson often written as Alfred Lord Tennyson?

Why is Lord Alfred Tennyson often written as Alfred Lord Tennyson? This occurs with and without a comma after Alfred: Alfred, Lord Tennyson and Alfred Lord Tennyson. Should Lord precede the entire name, and not just the last name?

The simple answer is...

Alfred Tennyson was created a hereditary baron, 1st Baron Tennyson. Barons are known by their title, Lord Tennyson, preceded if necessary by their Christian name.

The same applies to current Life Barons, who are not created with hereditary titles. Thus it's John, Lord Prescott of Kingston-upon-Hull.

Sons of hereditary peers are given a courtesy title. The sons of barons, that being the lowest order of the peerage, are simply known as The Honourable. Alfred's son was The Hon Hallam Tennyson until Alfred died, after which he became Hallam, 2nd Baron Tennyson, or simply Hallam, Lord Tennyson.

The eldest sons of higher ranks of the peerage generally get a courtesy title (the eldest son of a Duke will be granted the use of an Earl's title, and so on). Subsequent sons are known as Lord, using the family surname. The second son of one of the Dukes of Devonshire would be [for example] Lord James Cavendish. It's not correct to call that person "Lord Cavendish".

Thus it's possible to distinguish between Lord James Cavendish as coming from a higher rank of the peerage than Alfred Lord Tennyson.

For a more complicated answer with all the intricacies of the peerage, you will need to consult a reference like Debrett's.