Why did people in the 18TH century in the urban cities still use gas lamps

A late 18th century oil lamp supported on a wrought iron arch in Great Mary, eligible householders in the City of London were required to hang a lantern outside  Surviving examples can still be seen in many parts of Britain where they form part at the end of the century, but it was expensive, little used for street lighting.

where is the picture of the graph or table?

i normally just use