Lightning is not Bitcoin exclusive, Litecoin has it implemented too, basically any blockchain that has multisig and timelocks can utilize it. But I see what you mean, without the bloat of Lightning, Nostr can focus on doing one thing well, and that is messaging. Knowing the Unix philosophy, I should have known better :)
Discussion
The point is that:
Lighting network is for blockchains , high cost to maintain it.
Nostr is a simple protocol, is very cheap about cost to maintain and also simple just like access to the internet. To create a node in LN a normal people should study a lot. With Nostr not. It's completely different architecture. It's protocol like we use today https to read the website, or smtp for outgoing emails, or pop3 or imap to receive emails, these are protocols we use today. Nostr can have space in the future. Maybe. We will see. 😃
With Lightning you can mitigate that costs with collecting routing fees, with Nostr relay you need to charge for your relay and filter spam. I think it is easier to monetize a Lightning node