Because, it passive deal 5% attack damage based on target’s current health and the active let you life steal 15% of target’s maximum health plus 30% slow their movement speed for 4 seconds.
Other benefit of BotRK is a good item to kill dragon and baron, since both monster have high health.
* +25 Attack Damage
* +15% Life Steal
* +40% Attack Speed
Passive: auto attack deal 5% physical damage based on target’s current health
Active: Drain 15% target’s maximum health damage in physical damage and heal the same amount to caster, additionally slow target’s movement by 30% for 4 seconds
While it give some nice stats and ability to go against heavy health stacking enemy, at the end of the day BotRK do not give as much damage compare to BloodThrister which may give you higher damage in the long run as it give higher Attack Damage bonus for you auto attack and ability.