That’s because the G4 movie had a bunch of Hasbro product placement along with a story that seems like someone was told to write after only watching a brief synopsis of each of the characters personalities.
That, and a major problem g4 suffered from since the beginning was that whenever the stakes were raised beyond slice of life episode, the story had a nasty habit of becoming the Twilight Sparkle and friends adventure.
Also, the sudden inspiration for the solution via Pinkie Pie and “Pinkie, you’re a genius!” got really old really quickly.
Because the G4 movie was basically any regular FiM plot converted into a full movie while the G5 movie was a new generation of ponies, different plot, different ideologies, different style, different characters.
Also, because despite their best efforts, the people behind the g4 movie (whom I like and respect mind you) still made it so you needed a degree of familiarity with the show in order to understand what was going on. This meant that it ended up pleasing no one, because the fans didn’t like how they were getting a McCarthy-era FiM condensed into a film version that didn’t take into account the changes since Haber took over, and normies struggled to get invested, because despite the filmmaker’s best efforts, they still got a niggling feeling that “ok, if I watched the show I assume that would make sense?”
