One reason might be that, because the nonTB-version has only one fan and therefore a lower TDP, they couldn't use the same CPUs as in the TB-version. Remember that the 12" MB also hasn't been updated this year yet (and neither have we seen this rumored new entry-level MB/MBA), and Apple has said in the past that they consider the nonTB-MBP more as a MBA-replacement of sorts, so it's possible that they are grouping it together this year with the MB/MBA refresh in Fall.
Of course that's just one possible explanation; the other strong possibility is that they want to get rid of the nonTB-version in the long-term and have the TB across the board on the Pro-models.
In any case I wouldn't make any definite judgements just yet since we've only seen half of Apple's new laptop lineup of 2018 so far. Maybe we'll get an updated nonTB-model in Fall, or maybe they are replacing it by some strong MB/MBA models.
