Siri and Google Assistant, are just front ends to search engines - huge databases. Siri isn't falling behind, Google has unbelievable databases. (If you play trivial pursuit with a Google database, even if you're in a library and you're allowed to refer to books, you'd lose.)
Siri? Apple doesn't put that much emphasis on data collection, so they don't have as much data as Google. (Where "as much" is like comparing an ant to a nuclear missile.) Google's main business, remember, isn't cellphones, or Android Assistant, it's data. And they probably have thousands of times as much data as their closest competitor. And you can probably include the Library of Congress in that - as not even close to Google
So if you want to play "I have an AI on my phone" (you don't - you can do the same lookup as Siri and GA can), if you have an iPhone you're at a disadvantage. Unless Apple buys access to Google's data (which is as likely as seeing the moon doing the hula), Siri will keep falling further and further behind Android Assistant, until someone invents real AI that can fit into a phone. (Which probably won't happen in the lifetime of anyone alive at the moment.)