These Parshios speak about purities and impurities and the processes which are needed to purify oneself. In some situations, to completely get rid of the impurity, one needs to bring a korban, an offering. A convert also brought a Korban in the time of the temple to complete his conversion and ascend to a higher level. Nowadays, without the temple, this step is omitted for all these situations. When we stood by Har Sinai it was with korbanos, directly in front of Hashem. It was a singular event however it was supposed to carry on and be the Bais Hamikdash forever. As a result of sin, we were distanced from that level of the giving of the Torah and no longer have the opportunity to bring these korbanos and ascend to that great height.