This is a relatively recent trend, and it tends to be for anime where the premise isn't obvious before you watch it. In other words, it's typically to avoid spoilers. See, for example, Oshi No Ko, which bundled its first 3 episodes together and saved the OP reveal for the end in order to set up its premise.

I feel like the intuitive answer is that:

• The OP changes the pace/mood, so postponing it establishes the entire first episode as an introduction, much like the introductory pre-OP segments of any other episode.
• Suspense for people who are excited to hear the new theme
• More time for exposition if you only have 1.5 minutes dedicated to theme songs instead of 3 minutes
• Ends the episode on a high note to give a good first impression, since OP is usually higher priority/focus than the ED or have popular artists