There's a neat set of pros and cons for each unit type in the Battletech world.
Mechs are high impact, high value units. Like cavalry they are individually powerful but costly and must be used correctly with the right support to live up to their potential.
Tanks on the other hand are cheap and efficient, the workhorses of the military. Like spearmen. They are the smartest choice when garrisoning the vast territory you control. This can be further split into Wheeled, Tracked and Hover armor and the same with VTOL vs traditional Aerospace but yeah, lots of nuance when you dig into it.
Tanks also have the advantage that you don't need a highly advanced world to make it.
Many tanks (especially in 3025) still use good old ICE (internal combustion engines) with cheap ballistic guns.
Because knowledge and industrial base is still not as wide spread as they should be.
So while your planet might lack the knowledge to produce even the basic fusion engine (like us) but it can still slap together a Scorpion light tank with an AC/5 and a machine gun in record time (again like us).
Then you produce thousands of these and ship em off to various locales.
It also keep the peasants hired, so less chance of uprisings.
Of course there are exceptions like the infamous Alacorn tank which can even make the Clanners sit up and take notice.
When tanks meet high tech, they are almost as costly yet nearly as deadly as the most advance mechs.