Chalcedon had the chance to accept a more conciliatory 'from to natures' but they went with the more controversial 'in two natures' because they were forced by the emperor and because they didn't really like Pope Dioscorus.
Or because it's more accurate. 
2. When all had taken their seats in front of the rails of the holy
sanctuary, the most magnificent and glorious officials said: ‘Please make
known to us what you have determined about the faith.’
3. Asclepiades deacon of the great church of Constantinople read out
the definition, which it was decided not to include in these minutes.
4. After the reading, while some raised objections, John the most
devout bishop of Germanicia, {coming across to the centre,} said: ‘The
definition is not a good one and needs to be made precise.’
[...]
6. All the most devout bishops apart from the Romans and some of the
Orientals exclaimed: ‘The definition satisfies us all. This is the faith of the
fathers. Whoever holds a view contrary to this is a heretic. If anyone holds a
different view, let him be anathema. Drive out the Nestorians. This definition
satisfies everyone. Let those who do not anathematize Nestorius leave the
council.’
10. The most glorious
officials said: ‘If it seems good, let us – taking six
of the most devout Oriental bishops, three from the diocese of Asiana, three
from Pontica, three from Illyricum, and three from Thrace, accompanied by
the most holy Archbishop Anatolius and the most devout men from Rome –
retire together into the oratory of the holy martyrium. When they have
examined everything in order, their recommendations concerning the holy
faith will be reported to you.’
11. The most devout bishops exclaimed: ‘The definition has satisfied
everyone. [Report] our statements to the emperor. This is the definition of
the orthodox.’
13. The most magnificent and glorious officials said: ‘Dioscorus said
that the reason for Flavian’s deposition was that he said there are two
natures but the definition has
“from two natures”.’
14. Anatolius the most devout archbishop of Constantinople said: ‘It
was not because of the faith that Dioscorus was deposed. He was deposed
because he broke off communion with the lord Archbishop Leo and was
summoned a third time and did not come.
15. The most glorious officials said: ‘Do you accept the letter of
Archbishop Leo?’
16. The most devout bishops exclaimed: ‘Yes, we have accepted and
signed it.’
17. The most glorious officials said: ‘Then its contents must be inserted
in the definition.’
18. The most devout bishops exclaimed: ‘Another definition must not
be produced. Nothing is lacking in the definition.’
19. Eusebius the most devout bishop of Dorylaeum said: ‘Another
definition must not be produced.’
[...]
21. The most glorious officials said: ‘Your acclamations will be
reported to our most divine and pious master.’
22. Our most divine and pious master [=Emperor Marcian] has issued the following commands.
Either, in accordance with the decision of the most magnificent and glorious
officials, six of the most devout bishops of the diocese of the Orient, three
from Pontica, three from Asiana, three from Thrace, and three from
Illyricum, in the company of the most holy Archbishop Anatolius and the
most devout men from Rome, are to go into the oratory of the most holy
martyrium and produce a correct and unimpeachable definition of the faith
so as to please everyone and leave not a single doubt. Or, if you do not
approve this, each one of you is to make his faith known through his
metropolitan so as likewise to leave no doubt or disagreement. If your
holinesses do not want even this, you are to know that the council will have
to meet in the western parts, since your religiousness is unwilling to issue
here an unambiguous definition of the true and orthodox faith.
23. The most devout bishops exclaimed: ‘Many years to the emperor!
Let the definition be confirmed or we shall leave. Many years to the
emperors!’
24. Cecropius the most devout bishop of Sebastopolis said: ‘We
propose that the definition be read out and that those who dissent and will
not sign it should leave. For we are agreed with what had been well defined,
and raise no objections.’
25. The most devout bishops of Illyricum said: ‘Let those who dissent make themselves known. The dissenters are Nestorians. Let the dissenters
go off to Rome.’
26. The most magnificent and glorious officials said: ‘Dioscorus said, “I
accept ‘from two natures’, but I do not accept ‘two’.” But the most holy
Archbishop Leo says that there are two natures in Christ, united without
confusion, change or separation in the one only-begotten Son our Saviour.
So whom do you follow – the most holy Leo, or Dioscorus?’
27. The most devout bishops exclaimed: ‘We believe as Leo does.
Those who object are Eutychianists. Leo’s teaching was orthodox.’
28. The most magnificent and glorious officials said: ‘Then add to the
definition in accordance with the decree of our most holy father Leo that
there are two natures united without change, division or confusion in Christ.’
29. At the request of all, the most glorious officials went into the oratory
of the most holy martyr Euphemia together with Anatolius etc.
- Acts of the Council of Chalcedon, Richard Price and Michael Gaddis, volume 2, session 5, pp. 196-200
