That way, if someone wants to create an online Lux REPL for people to try Lux, it won't be hard for them to do so.
But I understand if you prefer a more permissíve license. Lots of people go with MIT License, it's very short, permissive and compatible with the GPL. Apache v2 is also compatible.
See here about EPL being incompatible: https://www.fsf.org/blogs/licensing/using-the-gpl-for-eclips...
Last time I tried understanding the key subtleties of the EPL I came out empty handed, but it would seem it's more viral than the permissive licenses above, bit less so than the GPL.
* That's why the AGPL exists, so someone working on a Lux REPL would only have to show the code for his online Lux modifications if Lux was AGPL.