I assume because you have to "claim" your node and not everyone has equally done that, not to mention it might not be good for privacy if you do, and some learned that after already claiming on one or another

Every node has a different view of the network based on the gossip it receives. nostr:npub19tcpurtt6xulhw0r6sc404j9jraj0h8me2lzs7z2tqewz7l0hpas59nlea uses several nodes to feed its database, which contributes to it seeing the highest amount of nodes.

Https://Amboss.space/stats shows the comparison over time.