A lot of old games were intended to be played in an endless cycle--arcade games especially--but in practice there was always some kind of limit due to the hardware and coding practices at the time. Pac-man, for example, hits a level where half the screen is the normal maze and the other half is a random assortment of other sprites. Donkey Kong ends when Mario always dies at a certain level.

The NES Tetris limit hit here is a point where there are certain "random" chances of the game crashing depending on certain states. In his run, Blue Scuti actually missed getting the first possibility of hitting a crash and had to survive a few more levels to get the next one.

This isn't the full ending, though. TAS runs show that it is possible to get up to level 255 and then loop back to level 0. Getting there requires missing every single possibility of a crash, though, and the probabilities of that mount up as you go.

Full breakdown: https://www.youtube.com/watch?v=GuJ5UuknsHU