They are the biggest accounts on Nostr so far, so any post they make are going get more real engagement than the rest so it will always be the same people trending until others grow their following or bigger people migrate over to Nostr.
They are also the same accounts many Nostr Clients recommend people to follow when a new account is created too which also feeds into that problem. If people want to find new interesting people on Nostr
nostr.band trending accounts for the day is the place to look https://nostr.band/. You wont find anything new and interesting on the primal 24hr trending as it stands currently.