I don't know why they can't simply work on the 2MB forked chain. It's not like they are stuck supporting a defunct 1MB chain if it goes that way.
Why do they need a 2MB block chain?
(so far the answer is just "because Gavin says so" and "maybe this is a popular thing")
Listen, I am not coming out and attacking you or giving you trash arguments, please don't assume the majority (as admitted by Adam Back) who want 2MB increases are uneducated and simply parrot what we hear Gavin or someone say.
Core ALREADY has come out in support of increasing block limit. The issue most have with this, is that it isn't quick enough.
Right now there is an artificial fee-market based off of the fact that Core Devs don't want to push out an increase fast enough. I think we can all admit, regardless of intentions or reasons, about the fact that this DOES benefit Blockstream (pushing users onto sidechains due to lack of ability to be included in the main blockchain due to block size) doesn't LOOK good.
We should have a block size increase because the fee market doesn't need to exist right now until block rewards are gone.
