Japan was a cash country but recently more people are getting Credit Cards. Also it wasn't so much people didn't have Credit cards but Japanese didn't like using their credit cards for online shopping. Using Credit cards online is getting more common now though.
co.jp can only be registered by a Japanese corporation & you only get one.
So when a company gets the first co.jp usually it's just their name. I don't think the transfer process for these would be so easy.
With dot jp you can buy, sell, register as many as you want.
In the eye of the consumer co.jp means Japanese company.
In the eyes of the Japanese consumer dot jp means a site for Japanese (I meant that statement to be broad)
dot com means international (even if their real target is still only Japanese).
Also most English sites selling jp are at a greatly inflated rate. Usually in Japan it's about $30 for a latin latter based dot jp & $6 for an IDN dot jp.
I always hear domainers say dot jps are worthless because they cost $100 to register. The registries in English charge $100 because their target is global business's, & frankly because they can...