It is easier to develop on an ip stack.
You have great tooling and great libraries out of the box because pretty much everything uses ip nowadays.
Plus, at least part of the controllers people use for their smart home will use ip anyway. A non ip network will need a bridge.
> How is that possible when thread use an ipv6 stack over 802.15.4 while zigbee use a simpler stack also over 802.15.4?
Easy, zigbee doesn't use a simpler stack. Using the same physical layer protocol doesn't tell you anything about the rest of the stack.
It's actually pretty simple. 6LoWPAN which is what Thread uses is more efficient than the Zigbee network protocol. Packets are smaller and the routing is better. It's not particularly surprising to be honest because Thread was designed by people who knew Zigbee really well and were aiming for an improvement.