Why wouldn´t you use the main blockchain for mid-to-large payments?
Because it can only process 7 transactions per second, world wide !! Pretty much NOBODY was even using it and fees already rocketed to $65/transaction. Now imagine that bitcoin ever becomes a success and most transactions > $150 still need to be processed via the main net. What do you think will happen?
Everybody here seems to agree, LN is ONLY suitable for small payments. But a system will never work if it can ONLY be used for small payments.