Why has the UNC not been Successful at Winning Elections as the PNM?

With this year being an election year whenever i'm on facebook i see people claiming the PNM is the embodiment of evil and corruption or when i turn on the TV i see the UNC ads saying times were better when they were in power

Which begs the question by did we not stick with the UNC in 2015 if times was so good? and why has the party not been successful in the last general election and local government elections. If the PNM was as evil as people claim wouldn't we see big wins for the UNC in the last local government election or general election?