Why do most modern fighter aircraft carry very less of armament -not about missiles- like the number of rounds in their primary guns eg 150 rounds for su-30mki?

In this particular case (i.e. the Sukhoi - and, coincidentally, the MiG-29) it is due to the efficiency of the targeting system - specifically the OLS-27 IRST: the better the system the fewer the rounds required to achieve a hit.

The designers actually stated at one point that, had they known how good the system was, they'd have halved the number of on-board rounds.

In addition modern rounds tend to be considerably more destructive than previous armaments, therefore fewer hits are required to bring down an enemy aircraft.