Great question! The effort was a contributing effort from all over Canada's eastern coast, but I can answer why the initial rescue effort left from Halifax. The answer is actually quite simple.

White Star Line needed two things immediately- a company with ships equipped for, and an undertaker prepared for, a mass casualty incident. Both of these were based in Halifax.

For the ships, they turned instantly to the Commercial Cable Company, who's telegraph lines connecting Europe and North America ended in Hazel Hill, Nova Scotia and who were based in Halifax. The CCC owned a ship specially designed to maintain telegraph cable connection - the Mackay Bennet- immediately contracted by WSL to be the first to retrieve bodies.

For the undertaker, they turned to the largest funeral business in the Maritimes- John Snow and Company also based in Halifax. By April 16th, Snow and Co. had contracted funeral professionals from Nova Scotia, Prince Edward Island, and New Brunswick. Even the largest undertaker in the province knew they were not prepared for the mass death that they were about to deal with.

Forty odd members of the funeral director associations headed to Halifax, requested to provide their own embalming tools, as John Snow began preparing the Mackay Bennet for body retrieval. This involved filling the Mackay Bennet's holds with ice, bringing aboard 100 caskets, and contracting the services of local faith leaders, Kenneth Hind and Rev. Henry Cunningham.

Shortly after noon on the 17th, the Mackay Bennet, transformed from cable ship to floating funeral home, left Halifax and immediately requested all local ships to report any evidence of the Titanic disaster they had encountered.

Bruce Ismay, managing director of the White Star Line -

The cable ship Mackay-Bennett had been chartered by the White Star line and ordered to proceed to the scene of the disaster and to do all she could to recover bodies and glean all Information possible.

Every effort will be made to identify bodies recovered, and any news will be sent through immediately by wireless. In addition to any such messages as these, the Mackay-Bennett will make a report of ijs activities each morning by wireless and such reports will be made public at the offices of the White Star line. The cable ship has been ordered to remain on the scene of the wreck for at least a week, but should a large number of bodies be recovered before that time she will return to Halifax with them. The search for bodies will not be abandoned until not a vestige of hope remains for any more recoveries.

The Mackay-Bennett will not make any soundings, as they will not serve any useful purpose, because the depth where the Titanic sank is more than 2,000 fathoms.

On Saturday, April 20th as the Mackay Bennet steamed towards Titanic's last position, liners Rhein and Bremen reported they had encountered bodies northeast of where Titanic sank, so M-B headed there.

The Bremen arrived and reported they'd seen over 100 bodies. First Class passenger Johanna Steinke describes-

As we drew nearer and could make out small dots floating around in the sea... looking down over the rail we distinctly saw a number of bodies so clearly that we could make out what they were wearing and whether they were men and women. We saw one woman in her night dress with a baby clasped closely to her breast. There was another woman, fully dressed, with her arms tight around the body of a shaggy dog*. The bodies of three men in a group, all clinging to one steamship chair floated near by, and just beyond them were a dozen bodies of men, all of them encased in life preservers, clinging together as though in a last desperate struggle for life. Those were the only bodies we passed near enough to distinguish but we could see the white life preservers of many more dotting the sea..

*The "shaggy dog" is a whole other Titanic mystery/myth. This is what Ms. Steinke believed she was looking at, but it's unlikely she was. But maybe!

As M-B steamed closer to the site, she began to receive more notice of body sightings. The Winifredian had seen wreckage only 25 miles from the wreck site. By the 21st, M-B arrived and began the process of retrieving Titanic's victims. Over 300 were pulled from the water in the span of 5 days, and had travelled over 170 miles from where Titanic had gone down. Absolutely overwhelmed by the amount, Captain Lardner of the M-B had buried 116 bodies at sea after mass funerals on deck of those who were too decomposed, unidentifiable, or crew. Identifiable first class passengers were brought aboard and embalmed despite condition.

The prayer-

For as much as it hath pleased Almight God to take unto himself the soul of our dear brother departed, we therefore commit his body to the deep to turned to corruption, looking for the resurrection of the body when the sea shall give up her dead...

Meanwhile, two more ships- another cable ship, Minia and a government steamship sent down from Quebec, the Montmagny headed out on April 22nd (again carrying John Snow) to pick up where M-B would leave off to sail home- packed full of remains. The final ship, the Algerine left from St. Johns, NF finding one last body and finally returning to St Johns in early June.

Over 200 bodies were returned to Halifax and stored in a curling rink, as people from all over the world made the trip to Halifax hoping to identify and retrieve their loved ones. Even President Taft sent a member of the War Department hoping find the body of his aide Archie Butt (he didn't). By the end of it all, 150 bodies were laid to rest, identified by name or number, in three cemeteries in Halifax- buried in a mass grave dug as a long trench.

By the end of it all, one body still lay unclaimed at the curling rink- a small, unidentified child estimated to be about two years old. John Snow paid for this child's funeral himself. The memorial was held at St. George's church with Kenneth Hind again presiding over his funeral. A white casket covered in flowers was taken to Fairview Cemetery and a stone erected, paid for by the crew of the M-B was erected and reading-

Erected to the Memory of an Unknown Child Whose Remains Were Recovered After the Disaster to the Titanic, April 15, 1912.

In 2007, the remains were DNA tested and revealed to be those of 3rd class passenger Sidney Goodwin, age 19 months.

Unfortunately, this wasn't the end. Bodies continued to be sighted and found throughout the summer of 1912. A Scandanavian immigrant ship related bodies hitting their hull and being sent flying into the air. Oceanic pulled collapsible A and three bodies out of the water on May 13th. The Ottawa found a body and buried him at sea on June 6. And finally- the Illford pulled the last victim from the water in mid-June, steward William Cheverton, who had been floating in the water for two months before he was found and buried at sea.

Sources and further reading-

On a Sea of Glass

Titanic: Triumph and Tragedy.

The San Francisco Call- April 22nd, 1912

Call of Duty: The funeral directors response to the Titanic disaster

Dundee Evening Telegraph, June 14th, 1912