It's because SNP put forward the original motion. Labour were suggesting a change to that motion ("amendment").
MPs are first invited to change the motion, then to pass whichever version becomes the final version.
Normally, this is a straight fight between the government and whichever opposition proposed it, but because Labour's amendment was also selected, it's possible (and in fact, the outcome) that the Labour amendment passed, so there was never going to be an opportunity for an SNP vs Government vote (even if the Gov hadn't pulled their amendment).
The sequence would be:
- Vote on Labour amendment
1a) If the amendment passes, the motion changes to the Labour version; then
- Vote on whether to pass that version (basically the same as vote 1).
If that passes, the Labour version carries.
Another possible outcome:
- Vote on Labour amendment
1b) Labour amendment fails.
- Vote on the original SNP version
If that passes, the SNP motion carries.
Another possible outcome:
- Vote on Labour amendment
1b) Labour amendment fails
- Vote on SNP motion
2b) SNP version fails.
- Vote on government amendment
3a) If that passes, the government version carries.
Another possible outcome:
- Labour version fails
- SNP version fails
- Government version fails
The motion fails.