I don't understand the conservative strategy on this at all

It didn't work during the 2015 election, so why would the whole selfie / just not ready work now?  Why would you double down on a failing strategy, especially one that becomes less true by the day?  At least in 2015 you could sort of make the case, but by 2019 he'll have been PM for 4 years so the attack is meaningless.  

The only way to get liberal types to vote CPC is to talk policy.  These attacks only play to the base - but the CPC can't win with their base alone.  It makes no sense.