because "X-rays" is such an UGLY word....
There's actually some truth to this. Originally it was called soft x-ray projection lithography. The other type of x-ray lithography was a near contact shadow technique using shorter (near 1nm) x-rays. To distinguish the two techniques they changed the name from soft x-ray to EUV.
This was also done for marketing reasons. X-ray lithography had failed (after sinking a lot of $$ into it), while optical lithography had successful moved from visible to UV, to DUV. By calling it EUV it sounds like the next logical step, instead of being associated with the failure that was x-ray lithography.
(Actually, x-ray lithography didn't really truly fail. It does work, but optical surpassed it before it was ready, so it became pointless)