There is no definitive answer to this question, rather there are accepted theories based on the available evidence. A variety of factors are generally cited. Among these are:
A) Britain was seen as the edge of the world. The acclaim and glory brought to the Roman general who could conquer Britain would enhance the prestige of the Republic, their family and their political prospects.
B) Rome had recently conquered Gaul (except for a small village of indomitable Gauls which held out under their chief Vitalstatistics...) and the territory was not yet fully subdued. Britain contained related Celtic tribes such as the powerful Belgae and Trinovantes, which could offer potential support and allies for Gallic rebellion.
C) Economic wealth... Britain had several valuable commodities; tin, slaves, silver and iron among them.
D) Generally antipathy to the 'barbaric' Celtic tribes and their religious practices.
E) a Roman expedition to Britain by Julius Caesar in 55 and later in 54 BC had pointed to the feasibility of invasion, and penetration/influence into Britain by Roman traders had made it possible to enact traditional Roman practices of dividing and conquering their targets
F) the Romans lacked truly good Fish and Chips shops
Because we were rich in precious metals
The Romans were cross with Britain for helping the Gauls (now called the French) fight against the Roman general Julius Caesar.
They came to Britain looking for riches - land, slaves, and most of all, iron, lead, zinc, copper, silver and gold
The Romans invaded Britain because they wanted to spread their Empires boundaries that is also why they fought Gaul and Spain.
Britain had no national identity as such at the time of the Roman conquest. The term backward is relative dependent on ones concept of civilization.
The failed attempt by Caesar in 55AD was the first stage in the attempt to colonise Britain. The Roman invasion of Gaul and the links with the Belgae who fled to Britain provded a spring board for cross chanel contacts.
The Iron Age people were formidable warriors but lacked the structure needed to defeat a proffessional, organised army.
The links with the Roman world went back for some time before the Roman conquest as is displayed by the grave goods found in rich graves in the the south of Britain prior to the coming of the Romans. Wine was a massive import in the south and luxury goods from the Roman world were sought after.The culture of the 'Iron Age' was rich in terms of beautiful art and metal work and advanced trading systems were in place. Britain was renound for trading in slaves, hunting dogs and tin. Some say that the south was 'softened to Rome'.
Claudius needed a propaganda boost to help his own position in Rome and the invasion of Britain helped provide this.
Beause Britain was not united in the national sense as we know it now, tribal unity was based on family loyalties and trading alliances and no united front was put up against the Romans.
Archaeological evidence, for example at the oppida at Maiden Castle shows that the Roman army was ruthless in it's methods and wholescale massacre was common place. As the Roman influences spread much more opposition took place and two noteable examples of this were the Boudiccan rebellion and the resistance in the area known now as the Marches. The Druids also posed a threat because of their overall influence on the population.
It was also probably part of the unstoppable expansion of Rome at this time.
The Romans invaded Britain because it suited the plans of Julius Caesar.
The republican general had conquered Gaul wanted an excuse to avoid returning to Rome.
So he seized the opportunity to mount an expedition on behalf of Commius. He wanted to conquer beyond the Great Ocean, and believed that Britain was full of silver and other valuables to be robbed.
They invaded Britain because the Roman's were cross with the Britain's for helping the Gahul's!
They want to get lot of money