Is there a good reason why modern airliners can't have conventional landing gears (vs. tricycle landing gears)?
Short answer: Yes. Not one, but several. A taildragger configuration for modern airliners comes with several disadvantages:


Visibility during taxiing is much worse.



Visibility while taxiing is a safety-relevant issue (Picture source)


Braking hard will result in a headstand. And with today's landing speeds, braking 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.



That is what is colloquially called a "headstand" (Picture source)


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 is 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.



Who says there are no taildragger airliners? (Picture source)