Jpeg is a format that you can edit in any picture editing software, and as long as the software is able to read jpegs, they can use jpegs, so it's very universal. Also, if they're rendering an image sequence, and a few frames are corrupt or if the render is truncated, they can easily re-render those frames. With QTs, they need to re-render the whole QT. Also, with image sequences, they can easily set it to render over a render farm. With QTs, it's not so easy.
MOVs require Quicktime support, and Quicktime runs only on Macs with limited PC support. Many codecs are not available on PCs. An example is ProRes. Although you can decode ProRes QTs with any machine with QT 7.6 installed, you cannot encode to ProRes unless you are running a Mac with Motion, Compressor or FCP installed. Another issue is the infamous QT gamma shift, which can be quite erratic across the board.
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