Comprehending why the Twitter limit exists should let you comprehend why there's no point for App.net to follow it.

The service won't actually be used for "micro-blogging" unless users are constrained by some limit, but 144 is too damn small. Nobody would ever have picked it if they weren't forced to, and these days they aren't.

What infinitesimal fraction of modern Twitter users in App.net's target market do you imagine use the service via SMS? The limit a legacy of an irrelevant use case.

I'd probably have gone a bit higher than 256 myself (disregarding attachment to powers of two), but it's an improvement.