Plus, you have to balance cost of the units + integration vs the cost of the purchase as a whole. These are expensive (even @ the rates RuAF pays, which are pretty excellent) large birds that can serve 30+ years.
Skimping on really (IMO) important defensive components that exist is a poor return on the rest of the purchase you have made. I'd understand if it was just the initial batch of 30 planes, or even the follow up, but at this point we are looking at 60 AF birds + another order for 60-75, and a Navy requirement of up to 50 air-frames. The time/money needed to integrate a defensive suit on par with Su-35 is a no brainer trade-off in my eyes.