Apple is in between a rock and a hard place on this. Webkit activity really slowed down a lot. Apple is pretty much the main contributor. It also gives them the most flexibility to incorporate into their products. Blink engine is very slow on the Mac/Apple products, and would put Apple at the mercy of Google ultimately. The Gecko engine is something they could probably use, but there are a lot of "mixed" opinions on how good the engine is.

Sidenote: I love Apple products, but to be honest Apple is not the best at making software. Recently, the quality has declined too. Their great applications are a few niche professional apps not designed for the average user. They have also gotten a lot slower at releasing updates.