Why are beetles so popular in Japanese culture ? and why do they keep huge beetles and huge crickets as pets in Japan ?

They are popular with children and it's not just any beetles, but 2 specific species called, stag and rhinoceros beetles. To understand why these insects and others are popular you have to understand that even though we do not know when it began, there is an almost tradition that young children, especially boys, would catch these beetles during summer and kept them as pets. It is almost a traditional childhood activity. If you were growing up in Japan, seeing other children collecting beetles, you would feel left out if you didn't. 

There are other reasons, too For example, most Japanese cannot afford or allowed by their apartments to keep pets such as a cat or dog.  Food being as expensive as it is in Japan, keeping a cat or dog means additional expense. And as small as the typical family home is, a pet is just not practical. But an insect like a beetle is easy to keep.

They love UK stuff

A beetle works much better than a Rhodesian Ridgeback in a Tokyo flat. The adults with the pricey beetles, are just buying their dream beetle they could not afford as a child. It's no different from the middle aged men who now drive the muscle car they wanted when they were 16.

They don't. Where did you get this idea? 

Grow big and eat. They are an insect-eating people.

Why?  No idea.  I thought they are popular with kids all around the world, not just in Japan. Crickets are nowhere near as popular with Japanese kids. Stag beetles are more popular and some rare kinds are traded at exorbitant prices.  My guess is that their popularity comes from the fact that they look like samurai warriors.