(i) TikTok's content is more engaging
(ii) Instagram's content could be easily recreated on TikTok
(iii) TikTok's explore feed is addictive
(i) TikTok videos are high-definition, fluid videos with catchy music. TikTok's compression algorithm almost seems lossless since videos are much more crisp than on Instagram/Facebook. I even believe they are upsampling videos from 30fps to 60fps (although this is just a personal observation and may be wrong). The catchy music is probably self-explanatory.
(ii) Think of typical Instagram content: an influencer posing or a travel snapshot. Now imagine the same content as short, crisp, and fluid videos with engaging music. Instagram content creators could easily recreate their content on TikTok in a more engaging way.
(iii) Upon opening the app you are shown the explore feed. You are shown a single TikTok video in fullscreen and you can swipe up to load the next video. The TikTok app seems to preload the next 5 videos which in practice means that content is always instantly available. The algorithm is quite quick in learning what content you like to see.
I think this quote sums up my thoughts quite nicely: "When I was a product manager at Facebook and Instagram, building a true content-first social network was the holy grail. We never figured it out. Yet somehow TikTok has cracked the nut and leapfrogged everyone else." — Eric Bahn, General Partner at Hustle Fund & Ex Instagram Product Manager
This is something I didn't think about: do they play video by itself and sync the audio to the video? They could then cache the audio once, and replay for all similar videos.