Obvious reasons
How about this reason: "Because I do not want a browser and on-screen keyboard".
It is rank raving lunacy to stick a browser on it (and a keyboard) when you have a 5in + slab to type and read on.
The value of the watch (or any other wearable that does not interfere directly with your field of vision) is in unobtrusive notifications, sensors and a set of simple UI tasks which specifically relate to these two categories. If you need to use a keyboard on a wearable, that means that the UI design for that wearable is a failure. This bit Apple got right (and so does Pebble and most of its apps). The current crop of Android smartware has this wrong. The level of wrongness is different - from Sony which tried, but failed to get it right (it was on the right track, just too early on) to Samsung whose Smart Watches are a complete idiocy as they are effectively a second phone - one you wear.
In any case, when you give it a thought, let's suppose we have the next wearable gadget - a wearable ring or an earing (there are use cases where making either of these smart may make sense). So, what's next - having a keyboard and browser on that too? Mobile radio? Wifi?