When the Bedrock server was made, the maps were all ports of Java maps. From my experience, it seems that the maps on Bedrock are some of the better and simpler Java maps.
I know SG the best, so I'll give an example. One of the Java exclusive maps is twice the size and scale of any of the Bedrock maps, and given the short nature of Bedrock games, would not be a good fit. Another Java exclusive map has a ring of lava around the middle chests, not to mention that the middle area is on a platform 50+ blocks above the rest of the map and is hard to get back to once the border closes in. These map features certainly would not be appreciated on Bedrock (I don't think they're liked that much on Java either tbh).
In skywars and eggwars, some of the Java exclusive maps have cramped islands that are quite easy to fall off especially if your movement is not snappy (e.g. mobile and console players). Also, in both of those gamemodes, maps are typically a lot larger and some are designed for teams of 3, 5, and 6, which there are not modes for on Bedrock.
To summarize, maps on Bedrock should be smaller and should be able to accommodate players with a larger range of abilities. Many Java maps not already on Bedrock do not fully meet these criteria. Although, if you see any specific Java map you think would be a good fit for the Bedrock server, you should make a suggestion post asking for it to be brought over.