This is for legacy reasons dating back to the time of the 8086 processor. Basically, the 8086 could only access 1MB of memory. In this one megabyte, the BIOS would store a lot of data structures (necessary for accessing disks and setting the video mode). When the first 32-bit processors came out, the original 32-bit operating systems still needed to make use of the BIOS in order to do things like change screen resolutions and enable graphics. To do this, they needed to leave the lowest 1MB of space intact and memory mapped to the corresponding physical addresses. Executables had to be based above this 1 megabyte in order to not overwrite the BIOS. 0x400000 is hexadecimal for 1 megabyte and so executables are often times loaded here.
Also, as the article mentions, other OSes like WinNT are capable of mapping executables within the first 1mb. Virtual memory and paging can hide the BIOS area away. I think your statement about "original 32bit operating systems" is inaccurate. The limitations given in win95 appears to be mostly due to win3.x legacy backwards compability.
In fact, the main article explains everything accurately. Your summary is more confusing than helping.
