It's part of the relentless push toward greater resource allocation efficiency. Just as it is orders of magnitude more efficient to distribute perishable goods through a centralized corporate supermarket chain rather than a patchwork of independent corner markets, Reddit reduces the total amount of operations overhead that once went in to maintaining thousands of independent vBulletin / phpBB / Discourse instances in the aggregate.
With all the recent talk about supply chain resilience and the inherent trade-offs necessary to improve it, one might wonder whether such considerations might be weighed differently going forward.