What was the reason the Romans left Britain the first time and not return till nearly a hundred years later?

The first attempt was a military one, which failed.

The second was partly military, but, partly civil. Being in the Roman Empire was a good thing, and outside areas often wanted in, to become client states. E.g., they would keep their Royalty, and other things, but, be part of the Empire as well. This was ultimately a very successful approach, in Britain, but, elsewhere as well.

In 55 BC, the invasion failed because the back up couldn't get through because of a storm.

In 54 BC the invasion was successful but the Romans were called back the Gaul to deal with problems there.

It was just a matter of time.... In 43 AD they invaid again - 100% successful

The so called invasion wasn't as wide spread or as big as some experts imagined/decided. Yes there were some big and small battles but overall the Romans wanted what they saw and mainly traded. There is evidence of miles and miles of streets with people from all over the world living, trading, farming and lots of shops, including Roman traders and shop keepers. [see Britain BC: 3 instalments] and Arthur's Britain for the best all round, comprehensive insight into pre-Roman and Roman Britain.

They drew Britain into the Roman sphere of influence by creating an alliance with loyal native chieftans and it suited the Romans to be paid tribute and have diplomatic links rather than go to the expense of stationing troops in Britain that could have been used elsewhere.

English cooking.