On the other hand, TAI does not take into account the variations in Earth’s rotation speed, which determines the true length of a day. For this reason, UTC is constantly compared to UT1. Before the difference between the two scales reaches 0.9 seconds, a leap second is added to UTC.
On average, Earth has been slowing down a bit over the past decades, so UTC is currently running 37 seconds behind TAI.
Reasons just states difference, but does not answer why.
There’s literally a section titled ‘why use UTC - not TAI?’.