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I think is a mix of features that made the videotoaster a good match for the Amiga.
The Amiga 2000 has an internal slot where both video signals and parallel port signals are available. This permits to Amiga and video expansion to talk conveniently.
Among the video signals there is XCLK which permits to sync the Amiga with an external clock.
Another important video signal is ZD which is high (or low?) when palette entry 0 is in use at a given pixel. This permits to the video extension to implement color key. The Enhanced Chip Set was even more flexible.
Amiga can show 4096 colors in HAM mode and animate graphics at reasonable speed.
Amiga can use overscan reducing the size of the borders
Sprites, Copper and bitplanes are extensively used to encode data towards the video toaster
The big thing is that the Amiga can natively work in NTSC. Don't know about early macs but for example VGA is totally incompatible with this standard so PCs weren't used that much for video editing in those days.