The common argument is that it is:

1) Advertisers won't give up tracking across the board. It's too lucrative for them.

2) Most users don't care about the setting that much.

3) Enabling it for default for the majority of users will cause present advertisers with a majority of users who they cannot track.

4) Per (1), the advertisers will then elect to ignore DNT.

5) DNT becomes a useless ignored flag and a waste of time.

Instead, if it's opt-in:

1) Most people don't bother turning it on.

2) The advertisers face pressure to comply with the ones who do.

3) People who do turn it on then enjoy the benefits.

Also, the DNT specification explicitly says that DNT should not be turned on by default, for these very reasons.