What are the other reasons that some believe that Christians will remain on earth forty days after the blessed hope?

Good question, and I believe this could be the way it is. It seems to have happened that way at the first resurrection:

Here is some scriptural evidence that convinced me:

Matthew 27:51-53 New King James Version (NKJV)

⁵¹ Then, behold, the veil of the temple was torn in two from top to bottom; and the earth quaked, and the rocks were split,

⁵² and the graves were opened; and many bodies of the saints who had fallen asleep were raised; ⁵³ and coming out of the

graves after His resurrection, they went into the holy city and appeared to many.

I tend to think when the resurrection happens at the rapture, the resurrected will go into the cities and be seen by many,

as that is what happened the last time.  After all, Ecclesiastes says:

Ecclesiastes 1:9

⁹ That which has been is what will be,
That which is done is what will be done,
And there is nothing new under the sun.

Later, scripture speaks of us living Christians as those who remain will be caught up.  Though this does not preclude the

resurrected ones from remaining as a witness which happened at the first resurrection, though they are caught up before we are:

1 Thessalonians 4:16-17 New King James Version (NKJV)

¹⁶ For the Lord Himself will descend from heaven with a shout, with the voice of an archangel, and with the trumpet of God.

And the dead in Christ will rise first. ¹⁷ Then we who are alive and remain shall be caught up together with them in the clouds

to meet the Lord in the air. And thus we shall always be with the Lord.

Additionally scripture that is often used to support that we are caught up instantaneously at the rapture actually is saying that

we are transformed instantaneously:

1 Corinthians 15:51-52 New King James Version (NKJV)

⁵¹ Behold, I tell you a mystery: We shall not all sleep, but we shall all be changed— ⁵² in a moment, in the twinkling of an eye,

at the last trumpet. For the trumpet will sound, and the dead will be raised incorruptible, and we shall be changed.

I have often thought that perhaps the following verse in Isaiah referred to our transformation at the rapture, and perhaps the

"walk and not faint", "run and not be weary" part refers to us being on Earth a short time before we are caught up:

Isaiah 40:31 New King James Version (NKJV)

³¹ But those who wait on the Lord
Shall renew their strength;
They shall mount up with wings like eagles,
They shall run and not be weary,
They shall walk and not faint.