Two reasons ... this winery is an afternoon thing, and I'm already set up for work that evening at the restaurant. I don't want to tear down my steady rig.
The other reason is the intimacy, and fullness of the keys. With guitar, I rely on more tracks, and rhythm for the ensemble, but in this intimate setting, a keyboard affords me options that will be more pleasing at a lower intensity. Tracks are awesome, in the right setting, but the quieter you play, the less convincing the music sounds when it's prerecorded. MIDI, MP3, drum tracks - they all suffer when you're not pushing a little more air. I can also sing a lot less with the keys, because I can fill in melodies to rest my pipes, which is paramount with this season's schedule. Singing 30+ hours a week can take a toll, when there's no down time to rest.
My job at the winery is background, not showman, so I'll use the tools that best facilitate that.
Every job is different. I've never been a cookie cutter performer, and I think that may be why I have been blessed with so much work. I tailor every job to the client, the venue, and the moment.