"why Lionel's price is so low."
As noted, diecast vs. plastic bodies lowers the cost of production. LionChief electronics use off the shelf parts, I believe, which almost certainly keep production costs substantially lower as compared with Legacy or PS3.
The LionChief system is a command system. You cannot control LionChief locos with TMCC/Legacy/DCS or conventional methods, true. However, LionChief locos work well, if independently with a separate remote, with layouts using other command methods or even conventional layouts, if you are careful .
It is easier and less expensive to integrate a LionChief loco into various layout approaches (DCS, Legacy/TMCC, conventional) than it is to integrate a PS3 loco into a Legacy/TMCC layout, or a conventional layout, as examples. You still have command capabilities with the LionChief, even on the conventional layout. With a conventional layout, you'll need DCS to get command capabilities of the MTH locos, and running conventional locos and PS3 locos on the same loop/block is not easily done. The LionChief loco will operate independently and well on the same loop/block on a DCS/TMCC/Legacy/conventional layout with any of those locomotive types. It's a feature, not a bug . Perhaps not a point relevant to everyone, for sure, but relevant to some.
My perspective is that from someone has ordered the LionChief models, and decided against the scale models. Also, I don't operate DCS, so that factors in my decision making.