why is e3 considered passive but e4 e6 not ?

1 e3 as played by Nimzovich is neither passive, nor defensive.
It is prophylactic.
Nimzovich played 1 e4 e6 and 1 d4 Nf6 2 c4 e6 as black, so he opened 1 e3 as white to play the same system as with black in reverse, with an extra tempo.

(@)tpr: 'Carlsbad 1929 was a top tournament.' ~ That was almost a century ago and was just one tournament.

'What is the use of looking towards the past? If past data dictates reality, then our 33,1 winning percentage last year means we should not even be playing [a different team] and their 66,7 winning percentage.' --- Toua from One Outs.

Taking century-old results at face value is dubious and undermines the potential value of data that is more modern, extensive, and reliable.

"century-old results "

• It is the same game with the same rules.
  Modern openings Berlin, London, Sveshnikov are also century-old.

(@)tpr: 'It is the same game with the same rules.' ~ But it has evolved in many ways: Strategies have changed; engines have become available; good openings have aged poorly (like the King's Gambit); bad openings have become good (like the Berlin, which was once considered to be dubious)...

'Modern openings Berlin, London, Sveshnikov are also century-old.' ~ And they have consistently performed well (and still are).

"they have consistently performed well"

• No they were forgotten until they resurfaced.
  Likewise the Vienna, the Scotch.
  I would not be surprised if 1 e3 resurfaced in the coming World Championship Match.