Does there need to be a why?
Seriously though Bamboo is a really good material for bike frames. A bamboo frame is around about the same weight as steel, depending on how it's constructed. Bamboo is a fast growing species of grass, so pretty sustainable, just the lug construction can be a bit environmentally dubious, again dependant on what's used. The frames themselves have really good riding properties, generally very stiff with a lot of vibration damping making them comfortable over long day rides, they are also infinatally customizable, meaning you can make any kind of frame you desire.
I decided to build one over one winter several years back and documented it all on here. Many folk on here build up bikes from raw frames, I just decided I would like the challenge of going one further and building my own unique, one of bike, including the frame.
As for longevity, don't worry there is a Bamboo bike over 100 years old in a museum in London and my own bike is showing no signs of age after four years, these bikes will easily last the same length of time as any other frame material. It is a shame you don't see more bamboo bikes, but don't write them off as some hipster fad, they are a more than viable alternative to the more accepted frame materials.
https://www.cyclechat.net/threads/self-build-bamboo-bike-frame-kit-build-thread.225144/