The truth is nobody here knows the real answer to that. The logistics behind why a certain supplier is chosen for certain components can vary greatly.

It could be entirely random where Valve approved those two suppliers for use on their products after doing some internal testing with multiple candidates. And depending on pricing or available supply, they could switch between one or the other with each new batch. This would allow them to keep selling units without facing supply shortage delays.

The other possibility is location. Perhaps the main plant that normally manufactures the Steam Deck doesn’t specialize in producing semi-transparent plastic shells, or they weren’t up to the level of quality Valve wanted. So they found a second plant to manufacture/assemble the LE model, where it just so happens BOE was the best and most cost-effective option for that region.

This is all pure speculation of course and chances are we’re never going to get the real answer behind it. Companies typically don’t publicly disclose detailed supply chain information. When that info does come out, it’s usually a result of consumers snooping around and finding patterns (like this thread), or the info leaked from court records in lawsuits.