There are a lot of people who just love Pokemon but there are some who started to not watch the series anymore after they have reached the end of the Johto Arc. Some say that the next episodes were composed of episodes that had no relation to the original plot of the anime series at all.
The fact that they cannot understand it can already make them feel bored with the series. Take note that not all episodes are filler episodes but they feel that the series is not a great as it used to be. This may have contributed to the further decline of anime in different parts of the world.
The reasons Pokemon fans dislike the anime after the Johto arc is because of the filler episodes. Filler episodes are episodes that do not further the plot of the anime in any way. Some common examples are episodes where everything ended up being a dream, or showing what would have happened had the main character made a different choice.
Some filler episodes are accepted and not all filler episodes that take place after the Johto arc are bad, the problem is that they happen too often. Pokemon was always rich with content and adventures and perils of the main characters, so the switch to filler episodes rubbed many fans the wrong way.