Because, in the 1990s, Pixar's IP was more popular than Disney's. The story (as I remember it from a Jobs biography) was that, in the Disney parks, there were longer lines for Pixar characters than Disney characters.
Someone in leadership (don't remember the name) basically swallowed pride and bought Pixar from Jobs. It was considered a "reverse acquisition" because Jobs had so much stock he technically controlled Disney afterwards.
This was certainly true for animated movies in the 2000s (where Pixar clearly dominated), although not the companies as a whole. Pixar shareholders (including Jobs) owned about 15% after the deal.
This isn't a reverse acquisition, it's just a normal acquisition. Company A (Disney) has many things but is missing one thing (an animation team that doesn't suck), so they buy a company that does have that thing.
Someone in leadership (don't remember the name) basically swallowed pride and bought Pixar from Jobs. It was considered a "reverse acquisition" because Jobs had so much stock he technically controlled Disney afterwards.