"There are some dozen small concerns
12 small companies
which left the States
left the States (the United States?)
for this business
for what business?
How do you "leave the States for a business?"
that have abondoned it altogether."
"that" seems to refer to the 12 small companies
Have abandoned WHAT?
One explanation is that some small enterprises went out of the business
Yes, it seems that 12 small companies went out of business
and handed it
Handed their companies? Handed themselves?
over to the government,
At least in the United States, you can't hand your business over to the government. The government in the US does not operate businesses because it will not compete with private enterprise.
A failed company may have its assets SEIZED by the government to pay for back taxes, but if that happens, the government sells the assets and keeps the money.
and they have given it up
"they" refers to the 12 small companies?
"it" refers to their business?
without room for regret.
I don't see anything in the sentence about regret or any other feeling.
That sentence does not communicate anything.
No wonder you don't have a clear idea about its meaning -- there isn't any.