A few reasons:

1.) I like the simplicity. No thinking about shifting. No grip shift to clutter my handlebar. No derailleur hanging off the frame. One less cable to deal with. Only one cog to clean.

2.) The chain is now higher up from the ground than previously with the derailleur, keeping it away from road crud; cleaner and thus easier to maintain.

3.) Hazards of commuting on the train - I had my derailleur knocked-in a few times, bending the derailleur hanger and throwing my shifting all out of whack.

4.) The drivetrain is oh-so-smooth and quiet. I'll never go back to a multi-geared clucker.

There are no huge hills in my immediate commute route that out-of-the-saddle cranking can't manage. I'll just need to detour the really big climbs, or think about getting maybe a 16T cog for those rare occasions.

Love your sig, btw.