The Cloudflare blog entries on TLS 1.3 have some commentary (and link to more commentary from other sources) about this topic. Besides just getting rid of legacy cruft, simplifying the protocol helps limit downgrade attacks, reduces maintenance burden, and limits the observable behavior that various implementations might end up inadvertently depending on.

EDIT: https://www.imperialviolet.org/2016/05/16/agility.html is a good post about this.