The Verse 33:6 must be understood as a reward for the Messenger of Allah and his household (his wives and children)... this means the honor for our Prophet applies for his wife too as they have been quoted in the same sentence and the words "النبي" and "أزواجه" are contented with the important letter "و" which in Arabic is called waw al-'atf واو العطف that means that what applies for the first (the word before it) applies for the second (the word after it).
Note that not any "و" or "واو" is of this kind!
In this summary of tasfir ibn Kathir you may read:
(and his wives are their mothers.) means, they are unlawful for marriage. In terms of honor, respect and veneration, it is not permissible for them to be alone with them, and the prohibition of marriage to them does not extend to their daughters and sisters, according to scholarly consensus.
Ibn 'Achour (or Benachour) in his tafsir at-tahrir wa tanwir added a narration of a saying of ibn 'Abbas at the funural of the mother of believers Maymunah from sahih Muslim to show this honor for them. And he also added that this verse isn't the one whom made them haram to marry afterwards but the parts of verse (33:53) who refers to this are:
... And when you ask [his wives] for something, ask them from behind a partition. That is purer for your hearts and their hearts. And it is not [conceivable or lawful] for you to harm the Messenger of Allah or to marry his wives after him, ever. Indeed, that would be in the sight of Allah an enormity.
The fact that they shouldn't be seen uncovered shows that they are not mahrams for men even if they are called mothers!
And here's a long quote from tafsir al-Qurtobi, which i will try to translate as good as i can in several parts:
وأزواجه أمهاتهم شرف الله تعالى أزواج نبيه صلى الله عليه وسلم بأن جعلهن أمهات المؤمنين ; أي في وجوب التعظيم والمبرة والإجلال وحرمة النكاح على الرجال ، وحجبهن رضي الله تعالى عنهن بخلاف الأمهات . وقيل : لما كانت شفقتهن عليهم كشفقة الأمهات أنزلن منزلة الأمهات ، ثم هذه الأمومة لا توجب ميراثا كأمومة التبني . وجاز تزويج بناتهن ، ولا يجعلن أخوات للناس ... .
(and his wives are their mothers.) Allah the Almighty honored the wives of his Prophet (prayers and blessings of Allah be upon him) by declaring or calling them as mothers of believers; in the means of the order of honor, benefaction, aggrandizement and the prohibition for marriage for men and they shouldn't be seen by them unveiled in opposite to (real) mothers. It was said that as their compassion was similar to that of a mother they have been put in the same position. This maternity doesn't oblige inheritance as it would be the case for as the (real) maternity. And it is allowed to marry their daughters and they shouldn't be declared sisters of people.
واختلف الناس هل هن أمهات الرجال والنساء أم أمهات الرجال خاصة ؟ على قولين : فروى الشعبي عن مسروق عن عائشة رضي الله عنها أن امرأة قالت لها : يا أمه ; فقالت لها : لست لك بأم ، إنما أنا أم رجالكم . قال ابن العربي : وهو الصحيح .
People differ on them being mothers for men and women or especially for men on two positions?. A-Sha'abi nararted on the authority of Masrooq from A'isha that a woman said: "O Mother." A'isha replied, "I'm not your mother but rather the mother of your men."
Ibn al-'Araby said: and this is the correct view.
قلت : لا فائدة في اختصاص الحصر في الإباحة للرجال دون النساء ، والذي يظهر لي أنهن أمهات الرجال والنساء ; تعظيما لحقهن على الرجال والنساء . يدل عليه صدر الآية : النبي أولى بالمؤمنين من أنفسهم ، وهذا يشمل الرجال والنساء ضرورة . ويدل على ذلك حديث أبي هريرة وجابر ; فيكون قوله : وأزواجه أمهاتهم عائدا إلى الجميع . ثم إن في مصحف أبي بن كعب ( وأزواجه أمهاتهم وهو أب لهم ) .
*I say (refers to al-Qurtobi): there's no benefit in making a restriction for men not taking in account women. And therefore it seems to me that they are mothers of the men and women; as honoring their rights towards both men and women which the beginning of the Verse shows (The Prophet is more worthy of the believers) and this covers both men and women by necessity. And this is what the hadith of abu Hurrairah and Jabir show. According this (and his wives are their mothers.) refers to both. Beside this we find in Mushaf Ubayy ibn Ka'ab (and his wives are their mothers and he is their father.).*
Al-Qurtubi quoted the hadiths of Jabir and abu Hurrairah in the context of the first part of the verse!
وقرأ ابن عباس : ( من أنفسهم وهو أب لهم وأزواجه أمهاتهم ) . وهذا كله يوهن ما رواه مسروق إن صح من جهة الترجيح ، وإن لم يصح فيسقط الاستدلال به في التخصيص ، وبقينا على الأصل الذي هو العموم الذي يسبق إلى الفهوم . والله أعلم .
And ibn 'Abbas recited (The Prophet is more worthy of the believers than themselves, and his wives are their mothers and he's a father for them). All of this weakens what Masrooq has narrated if it was correct (sahih) from the point of view of a superiority and if it was not correct than using it as a proof for a "restriction" fails and we settled on the the original case which is that this applies in general which is to be the first thought. And Allah knows best. END of the Quote.
To conclude the title was given them to honor them while honoring the Prophet in first place, because they brought to us many knowledge we wouldn't have as we don't know how our Prophet would worship at home or how he would treat his wives and kids or act as a husband, father etc. and many more only through their narrations. The prohibition to marry them is also a technical factor here but it was revealed in a different verse.
Note that -according the majority of scholars- this title applies only for those women with whom the Messenger of Allah consummated marriage before divorcing them ibn 'Achour here quoted the name of a woman from Kindah called Juwainah which apparently has been married later to al-Ash'at ibn Qais (apparently they married during the caliphate of 'Omar). A-Shafi'i said in a-rawdah it is haram to marry any woman our Messenger married even if it was only by contract.