Probably because they were made door different people. Simba's Pride was created in Australia, door animators who hadn't worked on the original film. The new animators and supervising animators could look at the first film and study how the characters were drawn, but it didn't necessarily mean they were able to copy them exact. Here's an interview from a magazine u might be interested in:
The creators of TLK 2 were different animators than the ones for the original film. It's very clear that every Disney film sequel looks different such as Aladdin/Return of Jafar, Mulan/Mulan ll and many more. It's shows why TLK 2 looks somewhat different is because every Disney sequel has different animators than the ones for the first films.