Why was the Ilbert bill was rejected by British parliament?

The Ilbert Bill wasn't actually rejected by Parliament as such -- it was never voted on in its original form. This was because it was so obviously unpopular from the moment it was proposed that taking it to Parliament would have been a mistake. Key opposition to the bill came from British people living in India who said that it would be humiliating for Europeans to have to face a white judge. More specifically, however, there was a lot of opposition to it from British women. Much propaganda was spread suggesting that Indian men were likely to rape white women and that having an Indian judge and jury would mean these rapists were likely to be freed. British women also contended that, because Bengali men habitually treated their wives badly, an Indian judge could not possibly understand a circumstance in which, for example, a British woman had been abused by her husband, because they would not see this as criminal, but rather would assume a man had a right to treat his wife in such a way.
At any rate, the bill was passed, but only after concessions had been made. These concessions stated that if the judge was Indian and the defendant was white, the jury had to be made up at least 50% by other Europeans. By this time, the bill was called the Criminal Code Procedure Amendment Act (1884).