In order to Mega Evolve a Pokémon, a trainer must acquire Mega Energy from special raid battles alongside other players. Unfortunately, the resource is being given out too sparingly, causing a large number of raids to be necessary in order to allow for a single Mega Evolution. Then, once you finally have enough energy to achieve the powered-up form, each transformation only lasts a mere 4 hours.
The Mega Evolved form being a short-lived transformation makes enough sense. The ability has always been a temporary upgrade in both the anime and games, but this is the first time we’ve had to expend such a cost to access a Mega Evolution. In canon, all that’s needed to Mega Evolve is a Mega Stone, and sometimes a significant bond between trainer and Pokémon.
Some players on the Pokémon GO subreddit suggest that perhaps the steep initial cost of Mega Evolution should be a one-off payment required to unlock the ability before allowing it to be used for an hour a day. Those who frequent the game’s subreddit have been mocking the update since it went live, referencing Mega Evolution as being a ‘Pay To Win’ feature.
The above image is in reference to the fact that raiding is the only way to currently obtain Mega Energy, and the process can only be done with a free Raid Pass obtained each day or by opening your wallet to purchase more passes. Putting easier ways to obtain the unique currency would be one way to appease fans but, as it stands the whole system could do with a rework.
Thankfully, Niantic claims to be actively listening to player feedback, and may well make some changes if Mega Evolution continues to be so negatively received.