Because it's everywhere, and it also can be measured to derive other units, like the Cubic Metre (or its more common non-SI derivative, the Litre).
Why not salt water at ocean-average saltiness?
Because then you need a way of accounting for the concentration of salt and anything else that might be present in your water sample, where as pure water can be obtained via distillation.
Why sea level?
Because its easy to account for without various atmospheric pressure measuring equipment
Convenient perhaps if you're working with scientific applications
That is kind of the point of having scientific units
It's also perhaps more convenient when dealing with air temperatures. 100 = "It's really hot out there", 0 = "It's really cold out there".
Now that really is arbitrary.