I don't recall seeing a definitive reason given but I'd always assumed it was because Xavier's patch was very large and the Ruby core team are usually conservative with these things. The solution they came up with was merely a few lines of C. It was a bit of a cheap hack but provided much (but not all) of the real world benefit, so they went with it as it was easier to audit.
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Some extensive background to the speedups that occurred in 1.9.3 with regards to loading so far along with the Xavier Shay work that didn't get included: http://www.rubyinside.com/ruby-1-9-3-faster-loading-times-re...
Why didn't it get included?
I don't recall seeing a definitive reason given but I'd always assumed it was because Xavier's patch was very large and the Ruby core team are usually conservative with these things. The solution they came up with was merely a few lines of C. It was a bit of a cheap hack but provided much (but not all) of the real world benefit, so they went with it as it was easier to audit.