Bleve originated as part of a solution to a problem customers faced when using Couchbase. Almost all customers have at least some sort of search use case, but often times that use case isn't particularly complicated. Many of them were running an ES cluster, moving the data from Couchbase to ES with an adapter, and using that to solve their search use case.

However, many of those same users complained about having to operate another cluster, especially ones that weren't already using the JVM (since it was a skill set they didn't have).

So, the appeal was to offer a service that runs as a part of the Couchbase cluster. It wouldn't have to match every feature of ES, just shoot for 80/20 and customers would likely find it beneficial.

It was fortunate that Go was still growing in popularity within Couchbase at that time, and we were able to position Bleve as a true open-source component, on top of which some money-making value add could be layered.

I can think of a few reasons. Primarily, this is easily embed-able. Not everything is a web-app/service.