Form factor and price of reader.  That's why I read a lot of books on a screen - but it's not a Laptop or a Tablet PC (though I have used a Tablet PC and worked well - just not great.) 

The device I use is an ancient (by techie standards) reader called the Franklin Rocket Ebook.  It was put out in the 1999-2000 timeframe and I managed to pick one up for about $200.  It's about the size of a paperback book and styled to feel like you're holding a paperback with the cover folded back to the left.  I get about 24 hours reading without the backlight - about 12-14 with the backlight on at the lowest setting (which is my standard mode.)  I upgraded it to 36 MB of RAM and never have run out of space on it.  It stores books in an XML format and does compression on the text to get the best use of space.  Text is low resolution and you don't want to put graphics on it (though it'll take PNG files) and the audible that it has is laughable.  But it'll take html and text files and let me take a library of text with me when I'm on the road - and for most trips I don't even have to bring my charger.  Powers up in about 5 seconds, and I could take annotations and upload them to my PC if I was so inclined (which I'm usually not.) 
THAT's the target (at least for the market segment of me.)  Give me that level of performance, but give me color, graphics, wireless connectivity, video (like I can get here, and (for some realism) the ability to read PDF's and I'm all for converting.  That's the potential I want the Tablet PC to meet - though I'm worried that we won't get there until the Organic LED's make the displays less expensive and less of a power draw.  The new Sony eInk book looks promising, but the eInk technology has some limits I'm worried about.