Why is it?

I have under the castoff pile in my room.
A TRS-80 model 100 laptop computer.
Tam reviewed one in her Digital Fossils blog.
With 4 old school AA batteries, that thing would go for 2-3 hours of Pre internet era computing.
Most of which is, thankfully, covered by the Statute of Limitations.
I expect new AA batteries, that outlast D cells, would power that old beast for 5-6 hours.
Why then can I not unplug my current laptop, and go into the rest of the house for more than an hour without that post-modern lithium-nickel-hydride battery pack begging the computer to demand I plug it back in?

Is this the progress I was led to expect?