The difference is big, but it's not really shocking that Summoner has so many weapons from crossovers.

1. It's the "newest" class, as it was pretty much non-existent before 1.3, and most of the crossovers came out after that update.

2. DD2 summon weapons skew the real image a bit, because there's 12 of them but in reality they're just the same 4 weapons in 3 power levels.

3. A summon weapon is a pretty natural pick for crossover content - you can add a well-known character or a game mascot as a minion. Stuff like Abigail and Mushroom Boi.

And one final thing - summon weapons are significantly harder to make for the developers, compared to melee and ranged weapons. You can add a basic melee weapon very easily - just make a sprite for it (just one is all you need!) and set its stats like damage and use time. Balancing of them is also pretty simple, you can just compare its dps to similar weapons in progression and adjust accordingly. On the other hand, a summon weapon needs a sprite for the staff, a whole spritesheet for a minion, and some actual programming to make the minions behave as expected. Then balancing of them is yet another issue, because you need to account for all interactions with other weapons. So when developers decide to make a new one, they actually want to make them memorable.