Because they need to do one thing and do it well. They don't have a 10000 person team to venture into "larger audiences", they do one specific thing or they die.

People don't wake up one day and think "oh hey, I'm in the market for a craigslist alternative, let's look for one". They need to be told that Yardsale exists, and it needs to find a place in their minds. The best way it can do that is to do one thing well.

As for why it's mobile specifically, look for why people do anything on their mobile phones.