LAS is primarily a leisure market, without as strong a business base as other traditional gateway airports like, LAX, SFO, etc.
Operationally speaking, operating long-distance flights into LAS is straightfoward--the problem is getting them back out. The combination of LAS's elevation above mean sea level, the 1% uphill slope of runway 25R, and the hot temperatures often make it difficult for domestic long-distance flights to make it non-stop, let alone international ones. So close to sea level, LAX and SFO don't have that problem.
Obviously, it's not impossible to operate LAS to overseas, as evidenced by the LAS-FRA service mentioned above. You can bet, however, that the "lid" for passenger bookings is either carefully monitored and/or controlled, or they're making tech stops somewhere along the way.
ALL views, opinions expressed are mine ONLY and are NOT representative of those shared by Southwest Airlines Co.