One and probably the primary reason for this change has been authenticity. Voted tickets even on the old feedback tracker asked for manual lasing and cool-down time.
Together with the addition of tank fire control-systems it made even more sense to make rangefinders operate manually. Otherwise we'd face a discrepancy between having a precise range - anywhere you look, all the time - with a rangefinder and having to press a key in a million-dollar tank cockpit (or less but still expensive RCWS or other advanced systems).

It's also an unification - with the updated Apex or Arma 3 key presets a single key - "T" - gives you a targeting action for the "object" under your cursor in multiple situations
- anywhere - reveal/report target to your group
+ Rangefinder - measure distance
+ RCWS - measure distance and adjust zeroing gun accordingly
+ tank and APC cannons - measure distance, speed, adjust zeroing and lead
+ vehicles or weapons with sensors - mark target
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