It's simply because Koichi is a very hard worker.
Koichi said, "To me, Tofugu is not merely for earning money. It's my life."
In kendo, it’s important not simply to defeat your opponent but also to train your own mind.
If you're moving towards (に) simple, you're saying simply or merely. This is one of those words that come at the beginning of a sentence. "Simply... "
If you know the readings of your kanji you'll know how to read this as well.