I think they're actually designated TMAs, not TCAs, and yes, the radar system is somewhat primitive. Actually only a few airports' TMAs have radar service. It's like ATIS frequencies and ILS systems in this country. There isn't precisely too many of them.
A typical Mexican TMA has a 50nm radius from the airport's VOR, and goes from the ground to FL200. That IS
And if you look at Mexico City... damn... Jeppesen even had to make a special airway chart for the Mexico City area. There are 3 main airports within 50nm (MEX
, by far the largest, TLC and PBC), so that makes the TMA more complex. Instead of having "independent" transitions for MEX
's STARs and SIDs, all aircraft overflying the area have to fly over all the VORs that are there for arrival and departure procedures in MEX
. For example, if you're flying from SAN
, you'll only fly over one VOR in the LA
or SXC). High altitude airways don't depend on low altitude VORs. But it's not like that in MEX
. If you're going from Dallas to Acapulco and flying over the MEX
TMA, you'll have to fly over the Pachuca VOR, then Otumba, then Mexico City, then Cuautla, and finally on to Acapulco. All that just to cross MEX
's airspace. All those VORs are used for SIDs and STARs in MEX
. The airway system over central Mexico is too centralized over MEX
. If you want to go from, say, Leon to Cancun, you have to fly over MEX
, there's not a "more direct" airway. I've almost never seen direct VOR-to-VOR routings on flight plans... I don't know why it's like that.
I hope I was clear. What I'm trying to say through this example is that Mexico's airspace strucutre could be a lot simpler and more efficient if the right resources were in place.
I'm no ATCer, so I can't give you the exact reasons for your question, but I hope my reply was useful in some way.
"... for there you have been and there you will long to return."