It was the cost/performance ratio, not of the CPU itself, but of the entire thing, including software. Sun sold highly performing, but extremely overpriced hardware that rode in on the hype it built around the architecture but failed to deliver on flexibility and bang for the buck.

I was involved in launching an ISP where the whole shebang ran on Sun boxes, and which was over-dimensioned to the point where I once stepped into the data center and found a waist-high box full of E250/E450... Feet. The purple plastic ones you had to remove to rack mount the things.

Two years later we'd shrunk down most of the whole thing (except the storage bits, where SPARC still had good performance) to a few racks of Compaq and Dell boxes that were vastly cheaper to maintain (both because they were cheaper, period, and because we didn't need to wrestle with Solaris and the compilers of the time to get stuff working on them).

This was back in 1999 or so, and I never saw a SPARC system in production after 2005 (until a few months back when I visited a telco customer who still swears by them for a very specific purpose).

I still have one of those plastic feet on my desk at home, as a reminder of the folly of buying single-vendor solutions. It sucks as a paperweight. :)