Why was my husband sent back to Mexico if he qualified under the 245i Law?

Your husband may have been qualified to obtain a green card pursuant to s.245(i)INA grand fathering provisions, but that law applies only to those who are physically present in United States. Once he was deported, and physically no longer here, he cannot avail himself of that provision. Mere filing of a petition i 2001 does not guarantee right to stay in the US and while he was in removal proceedings, he could have sought that benefit but as you indicated in your question, you did not receive an approval of that 2001 petition, hence the court's only option may have been to offer him voluntary departure. You should first inquire as to what happened to the 2001 petition. I fit is approved, then, seek to do immigrant visa processing at the US embassy in Mexico. I believe it is still possible for your husband to return to this country, but overall, once an alien has been deported from the US, the process of securing such person;'s return requires the assistance of a qualified immigration lawyer. Attempting the process on your own is not advisable. Finally, encourage your husband not to attempt to return to the US illegally without permission of DHS

He may have been eligible for 245i but if that was not told to the officer or judge or whomever removed him, then he was removable. You need to follow up with the agency or office that handled the removal.

If your husband was eligible for 245i, he should have made that claim in court. It's the responsibility of the respondent to inform the court as to what form of relief that he/she wishes to apply for with the immigration judge; the Department of Homeland Security will not make that determination for you. As long as your husband does not have any other issues of inadmissibility, you will now need to follow up on the I-130 petition that you had filed for him until he receives his visa interview at the US consulate in Juarez. After the interview, it's likely the consulate will require your husband to file an I-601 waiver application because he accumulated more than one year of unlawful presence before departing the United States. If the waiver application is approved, your husband will be allowed to return to the US immediately. If the waiver application is denied, he cannot return to the US for 10 years. In any event, he should NOT return to the US illegally.