Because Luke escapes Hoth undetected while the Falcon does not
At the risk of answering the question with the question, it seems that Luke just got lucky. After talking to Wedge on the planet, Luke is seen alone in space leisurely setting a new course to Dagobah:
EXTERIOR: SPACE -- LUKE'S X-WING
Luke's fighter, its wings closed, speeds away from the icy
planet. Soon it disappears into the stars.
INTERIOR: LUKE'S X-WING -- COCKPIT
Luke, looking thoughtful, suddenly makes a decision. He
flips several switches. The stars shift as he takes his
fighter into a steep turn. The X-wing banks sharply and flies
away in a new direction.
The monitor screen on Luke's control panel prints out a
question from the concerned Artoo.
LUKE: (into comlink) There's nothing wrong, Artoo. I'm just setting a
Han on the other hand is instantly in trouble:
EXTERIOR: SPACE -- MILLENNIUM FALCON
The Millennium Falcon speeds away from Hoth, closely
followed by one huge Star Destroyer and four tiny TIE
As it is pursued, the Falcon races toward two very bright
INTERIOR: MILLENNIUM FALCON -- COCKPIT
Inside the cockpit, Chewie lets out a loud howl. Han checks
as the ship is buffeted by exploding flak. He appears to be
doing six things at once.
HAN: (harried) I saw them! I saw them!
LEIA: Saw what?
HAN: Star Destroyers, two of them, coming right at us.
If Luke had left with Solo, then it is quite reasonable to assume that Vader would have had the ability to sense him eventually (this becomes more apparent in ROTJ) and pursue him specifically (whether he was in the Falcon or his own X-Wing). As it is, The Empire isn't even aware there is an X-Wing alone in deep space headed off to a remote planet inhabited by the galaxy's last Jedi Master. Vader's ability to sense Luke is not omniscient. In fact, it is outlined in ESB as very limited. He reaches out to Luke only when he already knows generally where Luke is and while he still close by ... and even then it seems a "loose" connection:
INTERIOR: VADER'S STAR DESTROYER -- BRIDGE
Vader stands on the bridge, watching as the Millennium
Falcon is chased by the TIE fighters. As his Destroyer draws
nearer, Vader's breathing gets slightly faster.
INTERIOR: MILLENNIUM FALCON -- SLEEPING QUARTERS
Luke realizes that Vader's ship is very near. He feels
resigned to his fate. He senses that he is beaten, more
emotionally than physically.