It's a combination of lack of redeyes and the fact that WN does not sell double-connects (except in some cases that involve a continuation flight with same flight number). The non-stop 5:20PM LAS-DTW flight is Sunday only and there are no LAS-DTW non-stop flights on other days of the week. The KOA-LAS flight arrives at 3:50PM which provides enough time to connect to the 5:20PM LAS-DTW Sunday flight. While the HNL-LAS flight arrives at 4:50PM which does not meet the 45 minute MCT for WN flights connecting from Hawaii to US48 destinations in LAS. It's possible that some same-day routings exist, but involve double-connects and thus WN will not sell them (it's possible to construct and purchase them yourself using multicity, but you will be purchasing multiple fares and it will likely cost more than single fare options presented by WN with a one-way search). For example, there's an HNL-OGG-LAS option that arrives at 4:20PM and provides an hour to connect to DTW flight which is a legal connection. There are also times when possible valid single connect options exist, but for whatever reasons revenue management has decided not to offer them for purchase on a single fare.