It's because a monotyped combat aircraft park is not appropriate for a war with an adequate enemy like NATO (20-55% less effective in hard risky missions).
There is a medicine proven fact, that special designed bomber will be rather more effective in low altitude deep strike, then a generic fighter. In this 20-55% of effectiveness sits the plane and crew life or death and a mission success or fail.
If you aren't going to fight NATO or US or China, it will be much mor cheaper to you to have monotyped fighter park of multi-rolled middle-weighted fighters like Rafale, Typhoon or MiG-29M/MiG-35.
Russia cannot afford itself to have 20-55% less effective fleet for the sake of budget.
However, Russians have developed not a single type, but a single family of heavy fighters - a T-10 platform. It allows Russia to have specialized fighters, bombers and interceptors which have 70-85% of similar standardized parts. This park is 20-55% more effective than a monotyped park of multirollers, but only 15% more expensive.
It is quite a reasonable approach as for me.
A fighter park for Russian Navy shore-based aviation shows complete different approach due to relatively small numbers comparable to VKS and a demand to serve all missions by one type even on the 20-55% less effective cost. So we see a Su-30 deployment to almost Navy only squadrons in Russia as a result of this.