Popular Vote Percentage:
2015:
2019:
The NDP did make some rookie blunders, such as the power fiasco. However, the main reason why the NDP lost a bit of the popular vote in 2019 was world oil prices. The economy was fubar'd and, as dumb as it is, political parties get credit or blame for the current state of the economy.
Meanwhile, the PC's and Wildrose merged and combined their votes. That "unite the right" thing really worked, in terms of winning an election.
Arguably, the UCP have made more and worse blunders in their term, and the worst ones may be yet to come. However, if oil prices remain high the next election will be the UCP's to lose. They do seem to be quite determined to lose however. Transmogrifying themselves into Wildrose 2.0 is bound to alienate a substantial number of the 54.88% of voters who thought they were getting the ol' PC's.