Deterministic lockstep. It's the legacy of Starcraft 2's engine. It allowed for rapid and inexpensive prototyping (for what was initially a SCII arcade game/map for Heart of the Swarm) and for the HotS team to focus on content creation but also provided us with the worst re-connect state out of any MOBA ever.
The more content and features they pile on, the less likely they'll address such a core issue with their game engine in the foreseeable future. Arguably they're so far past the point of no return and it will likely never be changed.
Edit: For funsies I've been playing my Shitty Uther since it's super late-night gong-show Quick Match.
1. Fist of Justice
4. Boundless Conviction
7. Holy Fire
10. Divine Storm
13. Holy Shock
16. Gathering Radiance
20. Divine Hurricane
Right click, stuns and nukes ahoy. It's not a good build if you want to, you know, heal your teammates, but it's fun as fuck deleting Valeera's in these uncoordinated matches with this build.