A taildragger configuration for modern airlines comes with several disadvantages:

 - Visibility during taxiing is much worse.
 - Braking hard will result in a headstand. And with todays landing speeds, breaking hard is needed if you want to fly from standard length runways. Carrier-borne aircraft were the first to feel this consequence of increasing landing speeds, so they converted first to tricycle gears.

![TBM-3U after having landed on wheels and nose][1]
That is what is colloquially called a headstand (Picture [source][2])

 - Loading and unloading will become more complicated. The current way of sliding containers manually on a flat deck would need to be replaced by internal winches pulling the containers up the ramp to the door.
 - Drag during ground roll is much higher as long as tail lift not sufficient to get the tail up. Take-off distances would also become longer.

In short, it comes down to minimizing field lengths and increasing safety.

  [1]: https://i.sstatic.net/XxabX.jpg
  [2]: http://replicainscale.blogspot.de/2011/08/walkin-dog-chief-goes-to-france-hawks.html