Why do creatures go back to a previous controller instead of their owner?
A change-of-control effect is a continuous effect (like a buf), not a one-shot effect (like a damage-dealing effect or moving a object)[CR 611.1, 610.1]. This allows the change of control to be temporary (e.g. Act of Treason).
Creating a continuous effect that changes the control of an object doesn't cause a similar existing effect to cease to exist. The effect of the existing effect is merely suppressed[CR 613.1, 613.1b, 613.2]. The older effect will reassert itself if-and-when the newer effect expires[1].
Effects that change text, set power and/or toughness, change types, change color and/or modify abilities work the same way. (For example, the continuous effect created by Turn to Frog does most of those.)


There are three ways a continuous effect can expire:

A continuous effect created by the resolution of a spell or ability expires when the specified duration has lapsed, if such a duration (e.g. &quot;until end of turn&quot;) was specified[CR 611.2a].

A continuous effect created by the resolution of a spell or ability expires when the specified condition is no longer true, if such a condition was specified using &quot;for as long as ...&quot;[CR 611.2b].

A continuous effect created by the static ability of an object expires when the object ceases to exist (i.e. changes zone)[CR 611.3b].






610.1. A one-shot effect does something just once and doesn’t have a duration. Examples include dealing damage, destroying a permanent, creating a token, and moving an object from one zone to another.
611.1. A continuous effect modifies characteristics of objects, modifies control of objects, or affects players or the rules of the game, for a fixed or indefinite period.
611.2a A continuous effect generated by the resolution of a spell or ability lasts as long as stated by the spell or ability creating it (such as “until end of turn”). If no duration is stated, it lasts until the end of the game.
611.2b Some continuous effects generated by the resolution of a spell or ability have durations worded “for as long as . . . .” [...]
611.3b The effect applies at all times that the permanent generating it is on the battlefield or the object generating it is in the appropriate zone.
613.1. The values of an object’s characteristics are determined by starting with the actual object. For a card, that means the values of the characteristics printed on that card. For a token or a copy of a spell or card, that means the values of the characteristics defined by the effect that created it. Then all applicable continuous effects are applied in a series of layers in the following order:
613.1b Layer 2: Control-changing effects are applied.
613.2. Within layers 1–6, apply effects from characteristic-defining abilities first (see rule 604.3), then all other effects in timestamp order (see rule 613.6). Note that dependency may alter the order in which effects are applied within a layer. (See rule 613.7.)