They remove it to promote other things, primarily for events. It does suck, but there are great alternatives;
Download a replay program (I suggest BaronReplays). In these they have a page that shows the featured games just like you'd see in the client, and you can watch them just the same. Even better, though, is that you can spectate other region's featured games too!
Go to a third party website, such as OP.GG and they host featured games as well, or replays of old matches which you can simply download and spectate (replay files are very small. They are not raw video)
they do it every time. i dont know why. its been requested in the past that you can get it back during an event. for a time theyve even payed attention to it.
i dont know why they went back to removing the spectator box. especially at events with champion reworks like this it can be interesting to watch games of the reworked champion.