What is the reason that a Convert is Allowed to Marry Someone who was Born Illicit?



Is a Giyur a normal jew like all other jew"s , if yes why is he aloud to marry a MAMZER, and why is he not getting a part of israil ?


To the Questioner, 

The Rama Mipano z"l (Ma'amar Ha'itim, siman 18) explains that Hashem has Rachmanut (Compassion) on the mamzer and is always looking for ways to bring him to tikun
and kashrut. 

Based upon this understanding, we can explain the following which will include a reason as to why a mamzer may marry a giyoret:

One of the ways that a mamzer's children can become totally kosher Jews is described in Mishna Kidushin (end of Perek 3).

Another way that allows a mamzer to lead a "normal" life which includes marriage and building a family, is his permissibility to marry a giyoret. The logical reason for the initial prohibition for a mamzer to marry the large majority of Jews, is based upon the principle: "Shelo Yehei Choteh Niscar" - that someone who sinned should not reap benefits. If the child who was born as a result of transgression would have all the regular rights of every halachically-born Jew, then the transgressors "did not lose anything". There must be a non- changing difference between a mamzer and others so that all transgressors will realize that their acts do have devastating effects. Therefore, a mamzer can in no way marry a regular kosher-born Jewess. 

However, the perimeter of this prohibition is mentioned explicitly in the Torah's wording of the prohibition: "A mamzer may not marry into the Kehal of Yisrael". Kehal is defined as all those Jews who were born out of halachically-directed kosher family set-ups.

Since Hashem is always looking for ways to help the mamzer and to bring him to tikun and kashrut, as mentioned from the Rama Mipano above, it makes sense that the restriction would only apply to the "Kehal of Yisrael". Although the prohibition for a mamzer to marry a regular Jewish-born Jewess must always remain without exceptions, as mentioned above, Hashem is still looking for ways to give the mamzer a chance to reach his potential, and so therefore, anyone not included in the strict definition of Kehal, such as a giyores, would now be permitted for the mamzer to marry.

It comes out that the rule which allows a giyoret to marry a mamzer, because "Kehal Gairim lo ikrei Kehal", is not meant to be said in a disparaging or condescending way; on the contrary, it is meant to underscore the fact that the giyoret would not be included within the prohibition that applies to the mamzer, thus giving her the opportunity to help and to heal in an area where others cannot.  
The fact that a ger does not receive a portion in the division of Eretz Yisroel is also not due to the fact that he is any less; rather it has to do with a technicality of how the land of Yisrael was divided - which also put limitations on regular born Jews from inheriting portions that they would have logically received had the land been divided according to a simple human-logic division. See Yerushalmi Bava Basra 22b as to the unique way that Eretz Yirael was divided. It necessarily involved taking into the calculation those who were present at the time of the Egyptian Exodus. Thus, one who did not have ancestors who were part of the Exodus from Egypt does not show anything less about him; it is simply a lack of fulfilling a technical condition which was required to be eligible for the division of land in Eretz Yisrael.