You may also have to consider the orbital planes of the planets. Now I really haven’t looked up anything deep about the Trappist system, but it may be like ours where planets orbit on similar planes, but not the same one. So while each orbital period is relatively short, it might take longer to get ones with good alignment between us and their star. Again, that’s just me guessing. I don’t have time to google it right now

In order to measure a planet’s atmosphere Webb needs to watch the planet transit it’s star (which provides light that filters through its atmosphere). The wavelengths of filtered light then tell us about the atmosphere (or lack thereof in this case).

This planet just happened to be transiting first, I’m sure they’ll look back and observe the others in the system when it’s their turn.