Simply because the SIP protocol looks simple and is easy to inspect... a lot of hardware/software will get it wrong. And I mean wrong like not implementing some feature on the phone thus bringing the common supported feature list down, implementing ICE in a wrong way then dropping the product support, breaking basic SIP transactions in a way that the phone doesn't even work without a NAT, etc.

Then router producers come in saying - we're doing home routers - let's help and solve it by doing SIP-ALG and active rewriting on our side. And they get it wrong again. And then they don't think of implementing a way to disable the SIP-ALG (looking at you speedtouch).

All the protocols are there. As long as you stick to one single manufacturer, it's ok. As long as you don't need custom features, it's ok. It's when you want to be good to everyone and support everything - then SIP networks fail because you simply cannot support everything at the same time.

Noone tries to analyse Skype and they control all the clients and servers. Whatever problems they create - they can solve. That's why they do better in many cases. Because you can't even "try" to fix their traffic.