Don't forget release-dates. If it's one of the first lenses released with IS, then it's likely to have not been as good an implementation as it is now.
The 70-200 f/2.8 IS (mk1) was 2001, after the f/4 (1999) and f/2.8 (1995), but that's when IS was just getting started, so maybe it was either rushed-to-market, or they just didn't know as much as they did now.
The f/4 IS was 2006, so it's a newer design, and by that time they'd probably ironed out all the creases in the IS.
The f/2.8 IS II is the most recent (2010), so it's had 10 more years of R&D compared to the f/2.8 IS mk1, and 15 years more than the f/2.8 original. The f/2.8 IS mk2 also has 1 Fluorite and 5x UD elements compared to the f/2.8 IS mk1 with only 4x UD and no Fluorite. It also looks like a complete redesign of the lens, the IS mk1 was probably just the non-IS with a few more elements jammed in for the IS, more glass is going to make worse IQ unless you change the rest of the elements to match...
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