Because..... The DS was my "gateway" bike and when I started riding was doing heaps of road riding and mixed riding and trails. Now I have a road bike the DS has become more specific for riding trails. At least once a week I do firetrails on my commute home ( a 3 km ride becomes 20km of trails with around 600m elevation )

While the 38c will certainly cope with the firetrails I do they are bit skinny and lacking in bite/grip especially on uphills. I'm not doing fast downhills or jumps or airs so the hybrid suspension is fine and with larger tires and lower pressure it'll be just that bit more comfortable and confidence inspiring.

Besides I can justify $80 for new tires and tubes to myself (and the Minister of Finance) easier than the cost of a decent MTB.

The DS still feeds my needs just needs a bit of tweaking to fit my needs slightly better.