salaries in Liga MX more competitive globally, compared to what they used to be. The peso has been one of the few currencies to appreciate significantly against the US$ and euro in recent years.

For reference, the peso was at MXP 25 per US$ four years ago, and it traded above 20 for several years. The current spot rate is MXP 16.79 (the strongest it's been like 9 or 10 years) so it has strengthened by about 33% vs the dollar in the last 4 years. Against the euro, the peso was at 27 four years ago, and it's now at 18.09, so also +33%.

This hurts the Mexican economy in some ways (notably manufacturing, exports & tourism), but it does make Mexican salaries more attractive relative to salaries overseas, from the players' perspective.

I recall something similar happening in Brazil a while back, when the real strengthened significantly and suddenly a bunch of the better Brazilian players returned from Europe to play in the domestic league.

I'm sure it's not the only thing going on, but it's definitely a factor, IMO.