StudyLight.org has a great run-down of commentators on this verse: https://www.studylight.org/commentary/matthew/11-14.html
Albert Barnes' Notes on the Whole Bible
This is a mode of speaking implying that the doctrine which he was
about to state was different from their common views; that he was
about to state something which varied from the common expectation, and
which therefore they might be disposed to reject
Coffman's Commentaries on the Bible
Basing their confident expectation of the return of Elijah before the
advent of the Messiah upon Malachi 4:5,6, the Jews of Christ's day
expected a literal return of the natural Elijah and had even tried to
shake the faith of the apostles in Jesus' Messiahship because, in
their view, Elijah had not yet come. Elijah did actually return and
met with Christ on the mount of Transfiguration (Matthew 17:3); but in
this passage, Christ revealed that the true intention of the prophecy
was not a literal return of Elijah, but his spiritual return in the
person of John the Baptist.
John Gill's Exposition of the Whole Bible
The words carry in them some suspicion of unbelief and hardness of
heart, as though they would not receive it: however, whether they
would or not, it was a certain truth, that this same person, "John the
Baptist", is Elias, which was for to come; who was appointed by God to
come, and was prophesied of Malachi 4:5 that he should come;
John Lightfoot's Commentary on the Gospels
The words hint some suspicion, that they would not receive his
doctrine; which the obstinate expectation of that nation unto this
very day, that Elias is personally to come, witnesseth also. Upon what
ground some Christians are of the same opinion, let themselves look to
Vincent's Word Studies
More correctly, Rev., If ye are willing or disposed. For there would
naturally be an unwillingness to receive the statement about John's
high place, in view of John's imprisonment.
Calvin's Commentary on the Bible
he glances at their hardened obstinacy, in maliciously shutting their
eyes against the clearest light.
Thomas Coke Commentary on the Holy Bible
There is nothing for the particle it in the original; therefore Dr.
Heylin reads him; and if this be the proper supplement, says he, we
must understand that total perfect repentance which constitutes the
common character both of him and Elias.
Johann Albrecht Bengel's Gnomon of the New Testament
It is your interest that is at stake. The expression, βιασταὶ (used in
the last verse), is explained: it is the willing only who are
compelled. All is prepared: it only remains that you should be
Schaff's Popular Commentary on the New Testament
The Jews expected that Elijah would rise from the dead, hence many
would not receive it. The popular notions on the whole subject of
prophecy were incorrect; for in the day of fulfilment our Lord thus
prefaces an explanation.
The majority of these examples are in agreement: It was against expectation that Jesus said this because of the common beliefs about Elijah's appear.
This is a parallel idiom to the more familiar "He who has ears to hear, let him hear" which follows in verse 15.
In Matthew 13 Jesus explains what he means by this type of idiom when he tells the disciples why he teaches in parables. He does so, quoting Isaiah 6:9-10 (emphasis mine):
Matthew 13:12-17 (ESV)
12 For to the one who has, more will be given, and he will have an
abundance, but from the one who has not, even what he has will be
taken away. 13 This is why I speak to them in parables, because seeing
they do not see, and hearing they do not hear, nor do they
understand. 14 Indeed, in their case the prophecy of Isaiah is fulfilled
“ ‘ “You will indeed hear but never understand, and you will indeed
see but never perceive.” 15 For this people’s heart has grown dull, and
with their ears they can barely hear, and their eyes they have
closed, lest they should see with their eyes and hear with their ears
and understand with their heart and turn, and I would heal them.’
16 But blessed are your eyes, for they see, and your ears, for they
hear. 17 For truly, I say to you, many prophets and righteous people
longed to see what you see, and did not see it, and to hear what you
hear, and did not hear it.
This shows that Jesus is speaking of spiritual things that must be spiritually discerned. Indeed, John the Baptist is the spiritual Elijah, not the physical person, and those with spiritual eyes (those willing to receive it) will see that they are receiving the fulfillment of the things they have been waiting for.