What was it about the Theodosian Walls made it so hard to take?

I read in this subreddit (please confirm) that even the Crusaders went after the weaker marine/sea walls.

This is correct, the Crusaders were able to scale the sea walls along the Golden Horn (see map below).

What was it about the Theodosian Walls made it so hard to take?

Constantinople was built on an extremely defensible position that was enhanced by the land walls. Look at this cool map. The city is bounded to the south by the Sea of Marmara, which has currents not conducive to amphibious landings. In the north, the Golden Horn harbor could be closed off and defended by a chain. The land walls themselves are quite difficult to penetrate due to being multi-layered (this is the Theodosian section). This is a restored section of the triple wall. They were built in response to the Hunnic invasions of vast but light armies without a siege train. The invaders from central Europe wouldn't be able to cut off sea supply lines if they attacked Constantinople so it was only necessary to make it too difficult for a land army to storm the city before an army could return and assist.

The towers of the inner and outer walls are staggered to provide good cover. Should an invader manage to penetrate the outer wall, they would still be under fire from the much more imposing inner one. See this cross section.

The weakest section was the Mesoteichion ("middle wall") due to its relatively low position. On the map, look for the river and the Mesoteichion crosses that valley. That's where Mehmed II concentrated his attack.