One of the meanings of 跟 (see pronunciation and definitions below) has to do with ‘following closely.’ When you add 不 and 上 after it, it means you can’t keep up. The sequence of tones (first tone ⇾ second tone ⇾ fourth tone) make it particularly easy to say and fit with the emotion of someone who is struggling with keeping up.

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Can’t Keep Up 跟不上 in Chinese