AeroMexico will fly a Boeing 787 four times per week between Mexico City and Los Angeles beginning in April, according to airlineroute.net.
Why use such a large aircraft on such a short flight?
A few possible reasons. One is that demand for travel between Mexico City and Los Angeles is high, and AeroMexico is capitalizing by offering more seats. Remember, this is a route that was once owned by Mexicana, an airline that no longer exists.
Another possible reason is that AeroMexico is seeking to funnel international traffic to Delta, which owns a small piece of AeroMexico. Delta does not have a huge operation in L.A compared to say, Atlanta, but I would imagine plenty of AeroMexico customers make a connection at LAX. AeroMexico could also be sending customers onto other SkyTeam flights at LAX.
A third possible reason is also the most simple. Sometimes, airlines have slack in their widebody schedules. Maybe the 787 arrives in Mexico City from Asia in the morning and does not go out to London until night. During the quiet period, it's possible to schedule a quick trip to Los Angeles.