It's Mech's Weakness

Mech players dying early to ground units either means they were WAY too greedy, or they were afraid of an air transition. Especially against zerg, the only early-game units for mech that shoot up are the Thor and the viking, both of which are expensive and not very good against well-controlled mutalisks. This is why mech isn't used often and you'll hear casters or players refer to it as very "unforgiving": mech is very weak to lair-tech units because an overcommitment to either anti-air or anti-ground will kill a meching player if the zerg decides to tech switch. 

In the games you're watching, the player likely made the mistake of delaying tanks because he overemphasized Thors - a lot of meching players automatically get Thors as soon as possible against zerg because if they don't, any number of mutas will kill them instantly, as marines are what kill mutas efficiently and the Terran won't have any of those. This auto-Thor strategy makes mech vulnerable to a lot of early-game roach attacks, because neither Thors nor hellion are good at all at killing roaches. 

And if they have tanks but not siege mode... that's just a mistake or an excellently-timed pre-siege attack by the zerg. D:

Hope I helped answer the question? :D