They were still selling the 3GS until just a few weeks ago; it would have annoyed many customers if iOS 6 wasn't released for the device. The first-gen iPad, on the other hand, stopped being sold when the iPad 2 was released in March 2011.

iOS 5 also doesn't run all that great on the iPad, so I'm sure that played into the decision not to offer iOS 6 as well. The 256MB of RAM paired with the large screen seems to be the biggest cause for this. The 3GS' hardware isn't great, but it's only powering a 320*480 display

I imagine they don't see it worth the effort to squeeze iOS 6 into the iPad's 256MB RAM.

(I know the 3GS also has 256MB of RAM but it's display resolution is much lower).

The ipad1 has a larger screen which strains the gpu more than the 3gs does. (that is the paraphrased reasoning iirc)

The 3GS effectively has better hardware than the iPad 1.

CPU, RAM, etc. are basically the same, but the iPad has to drive about five times as many pixels. Thus it effectively has much less RAM, an underpowered CPU/GPU, etc.

It's an odd and unintuitive situation, but despite being newer than the 3GS, the original iPad is ultimately a less powerful device, when it comes to which apps and OS releases it can support.

I'm sure Apple could support iOS 6 if they wanted to, but I imagine they decided that it wasn't worth the tradeoffs (e.g. poor performance, RAM available to apps, resulting stability or lack thereof).