Why do UCAV/UAV designs mostly use lambda wings instead of pure delta wings?
Aspect ratio.

Look at your two images: both wings offer the same wingspan, but the former requires a lot more wing area for it. Wing area is weight. More wingspan for less wing area means better fuel efficiency and thus better range.

Poor aspect ratio, leading to excess induced drag, is a major disadvantage of delta wings. Much of modern stealth design is an effort to mitigate the poor flight characteristics of early "stealth-only" solutions.

Neither UAV nor UCAV get meaningful benefit from supersonic flight, since they can't realistically oppose manned fighters. Large UAV and UCAV are meant for long-range predictable stealth missions, where slow and steady wins the race. Better fuel efficiency means more payload or longer-range missions, both directly improving the vehicle's usefulness.