Preference, mainly. We're Retell Certified Partners, and do a fair bit of enterprise-scale stuff on their platform. Multi-state agents are a big deal, and I believe those are still only in beta on VAPI (will stand corrected if those are live now!). Easy access to their dev team is also super important for us (e.g. had a super urgent issue today and got it fixed within minutes). Nothing wrong with VAPI either, though. Competition drives everyone to keep innovating and improving, and we all benefit from that.