These values were changed because in 5.x and beyond, search jobs started from the UI can now be queued when the search concurrency limit is reached instead of being refused. The back-end (splunkd) had this capability since 4.2, but the UI can only handle queued jobs since 5.0.
The bottom line is that in 5.x, the maximum number of concurrent searches has been lowered but it should be compensated by the full implementation of search job queueing. Overall, the goal is to improve the search experience on systems with high search concurrency: your search might get queued up for a bit (ideally, no more than a few seconds) but it should run faster when launched as there will be fewer searches contending for the same resources (most notably, disk I/O).
Further reading on this topic:
Accommodate concurrent users and searches
How saved searches affect Splunk performance
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