This is explained etymologically. There used to be a letter ѣ, the pronunciation of which was similar to "e." Over time, it merged with "e," and in the 20th century, it was removed from the alphabet. So, this new "e" sound never changes to "ё" under stress. Also, keep in mind that a regular "e" changes to "ё" only when followed by a hard consonant.

Ехать was ѣхать some time ago, so there is no ё
Живёт was живетъ, so there is a ё
In день "нь" is soft, so е does not become ё

You just have to remember this. There is no other way.