Because insulting Trump is a) easy to do and b) wins support from just about anyone except the most hardcore conservatives in Canada. That's why.
And guess what... the hard-right conservatives would never vote for Justin's party in a million years, so he doesn't need to worry about losing their support.
As for the trade agreements, that's called doing the bare minimum at work. We have thousands of individual trade agreements with the USA, all of which are constantly negotiated and renegotiated (and renegotiated some more), and Justin's job is to speak in favour of his country whenever any of them go public. These agreements stay in place no matter who won the last election in either country, and they'll be there long after Justin and the Bad Orange Man have retired. Or been kicked out of their respective offices, whichever comes first.