That's not to say that they don't have some other motive for the 90 day expiry, but I don't think they need the support of major CAs for what they're doing at the moment.
Even if they do get into Browser roots now, there are hundreds of millions of mobile devices out there that will not accept Letsencrypt without a cross sign. Lets face it Letsencrypt is dead in the water without Identrust (or someone similar).
Its not a bug, it's a feature.
This is important, considering that certificate revocation is not a universally solved problem, and that Let's Encrypt is aiming to radically increase the amount of certificate issues as a whole.
No strongarming necessary, I'm pretty sure technical considerations ruled the day here.
https://mozillacaprogram.secure.force.com/CA/PendingCACertif... shows Mozilla's in-progress certifications -- including LetsEncrypt, Amazon, DocuSign, VISA, a bunch of governments, telcos, existing CAs, and others I don't recognise. Cross-signing is a pragmatic solution for older client devices (eg. abandonware Android phones) for _any_ CA root, new or otherwise.