Because it is a proper noun rather than a common noun. Proper nouns, including names, are capiltilized to show that they are important.
Also this can be because people wish to contrast the one true God with 'the gods' who are idols and the inventions of men.
Paul G. Johnson has written: 'God and world religions' -- subject(s): Religions, God
G. G. has written: 'The divinity and humanity of our Lord Jesus Christ, proved and confirmed' -- subject(s): Attributes, Divinity, God
Robert G. Kirkley has written: 'By God, you can do it' -- subject(s): Christian life
F. G. Popov has written: '1920 god v Samarskoi gubernii'
Thomas G. Guarino has written: 'Fundamental theology and the natural knowledge of God in the writings of Henri Bouillard' -- subject(s): God, Knowableness
G. Etchegoyen has written: 'L' amour divin' -- subject(s): God, Worship and love
Iain G. Nicol has written: 'Vocation and the people of God' -- subject(s): Vocation
John G. Brandon has written: 'The joy ride' 'The big heart' 'Nighthawks!'
John G. Lake has written: 'Living in God's power' -- subject(s): Spiritual formation 'The original John G. Lake devotional' -- subject(s): Devotional calendars 'Adventures in God'
G. C. van Niftrik has written: 'Het bestaan van God in de kentering van deze tijd' -- subject(s): God, History of doctrines
Michael G. Brown has written: 'Kilimanjaro' -- subject(s): Anecdotes, Big game hunting, Safaris
G. Markham has written: 'An almanack for the year of our Lord God 1656' -- subject(s): Almanacs, English, English Almanacs, Ephemerides