That's because the PS3s BC was on a title-by-title basis, which leads me to believe they didn't emulate the PS2 per se. For example, and someone please correct me if I have this wrong, but I recall the Graphics Synthesizer (GS) having more bandwidth than RSX. So it's my suspicion that the titles made available as PS2 Classics were those that perhaps didn't make use of the full bandwidth available to the GS.
Be nice if anyone who knows more can fill in the detail here.
Besides, if PS2 BC had have been a trivial thing for PS3 then surely we'd have seen shit-loads of titles available. But there was hardly any. Given the PS2s vast and diverse library, there must have been a bloody good reason for someone to forgo all that low-effort easy-revenue. I mean, it was literally money for old rope.