Because he thinks he will be up in the polls even more once the vaccine train is at full speed and he's probably right with that. Look at the UK, where Boris Johnson got a serious bump in polls after the vaccination rate accelerated. Same goes for Israel and Netanyahu.

Currently Trudeau would risk another minority government. In two or three months, when vaccines are readily available for the whole population, he can paint himself as saviour of the people and the economy. The US-CAN border will be open again, tourists will flock back, restaurants will be packed ... and so on.

This way Trudeau can achieve a majority again.