Reddit does have the advantage of being a single site where search is easy to do in a shared db. Since Mastodon is federated, discoverability is going to be inherently worse as requests have to propagate through the network. But I think that for blogs it's less of an issue since you tend to follow people for their writing.

In my opinion, Lemmy is already a great replacement for Reddit. So, it makes sense for Mastodon to focus on its core functionality which is blogging, while Lemmy can fill the Reddit niche in the fediverse.