Because LINE does not allow it. A LINE group chat can contain only one Official Account, and every translation bot — LINE's own interpreter accounts, PlumiBot, and every competitor — is an Official Account. Invite a second one and it will not stay.
This is a platform rule documented by LINE for developers, not a decision any bot made and not something a setting can change. If you need a group to cover more than one language pair, the answer is a single bot that handles several languages at once — not several bots.
The reasoning is completely sensible, which is why so many people end up here. A construction firm has a group with a Taiwanese foreman, Vietnamese workers and an Indonesian supervisor. They find a bot that does Chinese and Vietnamese, add it, and it works. So they go looking for a second bot for Indonesian and add that too.
At that point the group stops behaving. Depending on what was invited and when, the second account is removed on the way in, or the group is left with one bot when it should have two and nobody can say which one went. It reads like a bug in whichever bot was added last. It is not.
LINE's developer documentation states the rule plainly: multiple LINE Official Accounts cannot take part in the same group chat or multi-person chat. That is the whole explanation. LINE enforces it at the platform level, above every individual bot, so it applies identically no matter which products you chose.
A few consequences worth knowing:
The single slot is not really a problem if the bot in it is not limited to one pair. PlumiBot was built this way: a chat switches on as many of 126 languages as it needs, and every message comes back in all of them together. The foreman writes Chinese once; Vietnamese and Indonesian both appear.
Setting that up is one command per language — the walkthrough is here.
If you are committed to bots that each do one pair, the only way to run two is to run two groups, each with its own bot and its own members. This works, and it is genuinely how some teams operate, but be honest about the cost: anything said in one group is invisible in the other, and somebody has to repeat every announcement.
The same rule is why you cannot put two bots side by side in one group and compare their replies. Make a second test group. It is a minor nuisance for testing and a fundamental constraint for real use.
If a group already has an Official Account in it and you want a different one, remove the first the same way you would remove a member: open the group's member list, find the account, and delete it. Once the slot is free the new bot can be invited normally. There is no need to recreate the group, and no need to contact anyone's support.
The one-Official-Account-per-group rule is documented in LINE's Messaging API documentation for group and multi-person chats. Reports of exactly what happens to the second account vary — what is consistent, and what matters, is that it does not remain in the group.
PlumiBot