Why do Hindus class Sikhism as a Sect of there Religon?

Sikhism has nothing to do with Hinduism so why are some Hindus saying that Sikhism is a apart of Hinduism , when its Not apart of Hinduism or Islam its completely Differant religion.

Because Sikhism has its roots in both Hinduism and Islam, just as Christianity has its roots in Judaism. Sikhism isn't terribly similar to either religion, other than being a monotheistic religion like Islam and unlike other religions from the Indian subcontinent; but at the time of the arrival of Sikhism, Hinduism and Islam were the two predominant religions in the area of Punjab, where Sikhism originates from.

Because they do not want to give rights to them. They even
declared Muslims as one nation with them, but they did not
trust them and got an independent country Pakistan. Sikhs
are the only religious group who are still devoid of a country
of their own, even 63 years of independence. Budhs got a
country, Muslims got a country, but Sikhs are living as a
hapless minority in India today.

M J Iqbal

They have tried many ways:

Guru Amardas was reported to emperor Akbar by his minister Birbal(a Brahman), Guru Amardas sent Bhai Jetha(later Guru Ram Dasi) to explain every thing in Akbar's court. Akbar was fair and he listened and objections were overruled. One of the objection was, Guru does not follow Vedas and Verina system and the answer was " We are not Hindu and thus we are not required to follow them". Birbal also tried to impose tax on darbar Sahib, when Guru refused, he threatened to destroy it. Fortunately he was sent to a war where he died and did not come back.

Many times Hindus raised questions. Greedy Hindus got benefit when mughals placed reward on reporting of every sikh, many Hindu's name was in the top list who received rewards.
There is a class of Hindus who are always hostile to sikhs but there are many good Hindus, who helped the sikhs during trouble period. During Gurduara control movement started, many Hindus got baptised and joined sikhs. Presently there were many Hindus who joined agitation under sant Jarnail Singh. Waheguru Singh died fighting with Indian military in darbar sahib, he was converted from Hinduism. Many members of Sarpanch Bihari Lal died fighting for sikhs in the struggle after the attack. Roshan Lal Bairagi(later Gurdev singh) was a well known militant leader.

There is no reason, it is just a criminal activity. Dr. Iqbal writes regarding Islam, " The religion which has conquered every corner of world, sinked in the ganga of brahmanism". Iqbal was one who was part of muslim revival movement.
There is no code, people who sacrificed everything for humanity are equated with normal humans. Guru Gobind Singh was equated with Shiva jee Marhatta. Whereas Shivajee was a king and a politician, guru gobind Singh was a saint. Shivajee even attacked hindu temples of opposite tribes, he believed in caste system but Guru Gobind Singh declined separate baptism to Hindu Hill Rajas who ultimately invited suba sirhind and aurangzeb. Satyarth Parkash's writer gave very insulting remarks against sikh icons like Guru Nanak, Guru Gobind singh and bhagat Kabir and arya smaji Punjabis without any reason gave him status of maharishi. One such man, Gurudutt even gave higher status to him than Guru Gobind singh. His remarks were very insulting.

Hindus even opposed Punjabi language and declared there language Hindi just because sikh scriptures are written in Punjabi. In punjab Hindu schools still give first preference to Hindi, a reason best known to them.

I think problem also lies with sikhs too. It is a big community but you can not count on fingers, the people who are well known for explanation of sikh philosophy. Western sikhs though less in number but they know philosophy of gurus.

I agree with answer of Javed Iqbal and steel boy .

Brahmins destroyed Buddhism 1000 years back

Because Guru Nanak was so divine, so perfect a human being that they want to claim him as their own. I think for the same reason Muslims, too, were making such claims. As a person who happens to be Muslim I have much love and respect for him and believe that he was completely in the service of God, wage Guru.