When v7 would just have been a move to a new kernel ...
It's pure speculation, but I think that "just switching kernel" might be the problematic part. If they modified too much in the old one (and it looks like a lot of what RouterOS does is not just in userspace), porting everything forward can be hard. Linux is evolving, and it's being worked on by many people, much more than MikroTik has (I guess). So they can either break from it, go own way, and lose all those nice improvements already done by someone else. Or they can try to keep up with changes, port their stuff, but that can be an awful lot of work.
I'm not sure about that. The drivers are kernel bound, of course. So are the NV protocols - with all that TDMA and whatnot. But (almost) everything else? Is just userspace. Even routing: you change it in linux with a userspace command. The kernel may change as much as they want - the interface is still the same.
Even things like L2TP shouldn't be a problem: You still talks to the kernel through a userspace interface. IPv6, PPP, OpenVpn, IPSec... everything is userspace controlled. Yes, yes. OpenVpn creats an interface (tun or tap). They are provided by the kernel, and are userspace controlled. If the Linux kernel was ported to a given architecture, chances are they're already available.
The drivers... well, the drivers are the hard part. If they aren't part of the kernel three, then Mikrotik must get them, compiled to the new kernel, from the hardware makers. And this is not always easy... Then they need to test, test, test, integrate, validate... a truly hellish task.
I agree with what was said: they promised too much, and now are trying to live up to their promises. This means a lot of development, in a lot of packages. And the drivers! Have I said how hard the drivers part will be?