Because we are looking at another potential impact just two days ago on August 20th. This report comes from Junichi Watanabe of Japan using a fairly modest 6 inch telescope and a decent webcam.
So far, no one else has reported this impact and initial follow up observations reveal no impact scar on Jupiter. If it was a small impactor and burned up higher in the atmosphere, it is possible for there to be no impact scar (as was the case with the previous impact).We still need a lot more data, but if we keep finding them at this rate, we might have to revise our expected impact rate upward a bit (at least for Jupiter!)
Reprinted with permission from the Half-Astrophysicist Blog.