Massing trophies is difficult as you start out the game. Likely you will have played mostly one character, gotten good with them and stuck with them to try to be the best you can with them. This is usually a good strategy to win a lot of games in a row and increase rank in multiplayer games. However with Brawl Stars, their trophy system works a little differently, you gain more or fewer trophies depending on how many trophies that brawler already has.
So the higher rank and trophies that brawler has, the lower the trophies gained for each win, whereas the lower the trophies on that brawler, the more trophies you will earn per win. The second factor that plays a part in your speed in gaining trophies is the level of players you play against. The higher rank the brawler you are playing determines the rank of the brawlers you play against. So a high-level brawler will play against a team of a high-level brawler, and a low-level brawler will play against low-level brawlers. This keeps the game fair, but it makes earning trophies at high levels much harder because you will also be playing against much higher level opponents, and wins will come less easily. These factors in total mean that the fastest way to gain trophies is not to be good with one brawler and grind up a thousand trophies on him, but instead to play all of your brawlers. It will be much much easier and quicker to gain 200 trophies across 10 brawlers against easy opponents than to grind 2000 trophies on one brawler against tough opponents.