why such power sequencing is needed ?
Because the datasheet says so.
Seriously, this is actually the only relevant answer. It's not your business to second-guess the datasheet. If it says you need to do something to make the device work, then you need to do that if you want the device to work. Guessing why the manufacturer says something is necessary, then not completely following the requirements because you think you know better is irresponsible design.
One possible reason is that there may be a latchup problem. If power is brought up in the wrong sequence, then maybe some parasitic SCR or something gets turned on, a lot of current is drawn when the remaining supplies are brought up, the poof.
However, again, it's not your business, other than just curiosity, why a spec is what it is. It's a spec, a rule. Follow it or else. If it's too inconvenient, then find a different part that doesn't have such a requirement.