The DLCs are simply harder.
For one, it's common knowledge that each of the NPCs has a weapon type it's not super-friendly with. For example, Dead Money hates gun users, Honest Hearts hates energy weapons, Lonesome Road DEFINITELY hates unarmed and melee, and Old World Blues is kind of a struggle if you use anything but energy weapons.
Second, each has it's own challenge. Dead Money is a puzzle challenge and the Ghost People wielding throwing spears are definitely dangerous. Honest Hearts in theory isn't hard, but the enemy spawns are SOOOO [censored] that you end up with three giant Yao Guai suddenly spawning directly in front of you. Old World Blues is FO3 style difficulty; the enemies aren't particularly deadly or dangerous, however they ARE bullet sponges so they really press your resources. Finally Lonesome Road you....just try not to get friggin' hit in Lonesome Road.
Finally, enemies in the Mojave generally don't scale to your level. The DLC enemies do, to an extent. Not completely, but they do. I believe Honest Hearts enemies will scale all the way to level 40, Dead Money enemies can reach 45, OWB scales til 36 or so and Lonesome Road enemies scale all the way to 50.