A bot developer could find it difficult to maintain a bot on different channels.
Currently it’s straight forward to develop a bot and distribute it on Facebook, Telegram and Wechat but if you want to plug that into Alexa or Siri then it’s more difficult.
A platform (like Alexa) needs producers/content/functionality which the 3rd party bots bring. Attracting these developers is part of the battle.
If there are breakthrough bots you want to make it easy for them to be on your platform.