Let's all say it together, the difference with Android is...
..."ANDROID IS OPEN SOURCE!"
That's the main reason behind, well, everything. They make the OS in itself, and put it on their Nexus (or close to Nexus) phones - they do not put it on HTC's phones, or Samsung's (developer edition Galaxy and One excluded), the manufacturers do it, how they want, when they want. This gives you the choice for many different potential experiences and UIs based on what you think will work best for you. Delays come from carriers, and manufacturers trying to get you to buy a new phone.