Why did early ASCII have ← and ↑ but not ↓ or →?

In the Atari 2600 cartridge, BASIC Programming, ← is the assignment operator, rather than "=".

https://en.wikipedia.org/wiki/BASIC_Programming#Sample_code

The cartridge requires Atari keypad controllers, and was bundled with overlay templates to indicate the key functions.

https://boston.conman.org/2015/06/16.1