If a company were capable of paying 100% in cash for another company, why would it choose NOT to do so?

It might be saving its cash for something else or it might be concerned about running low if business takes a turn for the worst; its stock may also be trading at an all-time high and it might be eager to use that instead (in finance terms this would be “more expensive” but a lot of executives value having a safety cushion in the form of a large cash balance).