As you said, LCD and DLP based RPTVs can only do native 720 progressive (1280x720 pixels)* HD signal only, and are not capable of displaying native HD 1080i (1920X1080 pixels)**. But CRT-base RPTV can do 1080i native HD signal.

Is "fixed" pixels the limiting factor for LCD&DLP displays, or is it some other reasons?

* 1280 pixels horizontally, and 720 scan lines vertically.
** 1920 pixels horizontally, and 1080 scan lines vertically.