Answer:
This is because there is no problem of heating up of the atmosphere around the shuttle at takeoff, but on low-level reentry, the atmosphere around the shuttle is heated up.
Step-by-step explanation:
When a rocket takes off, it is relatively travelling at a lower speed at low-level atmosphere where the atmosphere is densest. This slow speed means that the air drag on the shuttle is not too much that the air is compressed and heated up. The result is that not much atmospheric heating is encountered, as the rocket's speed accelerates slowly and is only very fast at high-level atmosphere where the air is thinnest.
For low-level reentry, the shuttle is accelerated from outer space, under the influence of gravity, and has reached a sufficiently high speed at low-level atmosphere where the air is densest. At this level, the atmosphere immediately in front of the shuttle is greatly compressed. The effect is that the compressed atmosphere becomes exceedingly hot, and ignites into a burning flame in front of the shuttle. At this stage, the thermal shielding tiles on the wings of the shuttle is very important, and a damaged tile that was no problem on takeoff becomes a problem at this stage.