Same reason we always changes laws and/or amend the constitution - because it makes sense and is consistent with the circumstances of the present (as opposed to what was relevant 250 years ago).
For one, life expectancy is considerably longer today than what it was 250 years ago. In 1776, the life expectancy of an American male was 38 years, which would make the concept of a lifetime appointment to any job very reasonable. Today, life expectancy is 78 years, which means a judge can sit on the bench for many decades which doesn't make any sense in terms of getting new blood onto the bench relative to how much power a SCOTUS member wields.
The second reason is that nominating SCOTUS judges has turned into a political circus specifically because they are there as lifetime appointments and each side is incentivized to fight as hard as possible to undercut and discredit the opposite side's nominations because they know how much is at stake by way of lifetime appointments.
There are two ways to fix this - one by what the Dems just proposed (which isn't likely to happen given it requires Constitutional ratification), and two by simply increasing the amount of SCOTUS judges to 13, it will allow more judges to cycle in and out of their positions and more Presidents to nominate new judges.