The answer is a very simple one told by Charlotte to Elizabeth in her own words, though the language of Austen's time, being more elegant and complex, may be hard for contemporary English speakers to understand. For background, Elizabeth muses, after hearing Charlotte explain that she has accepted Collins' proposal of marraige, that she always supposed that she and Charlotte had different views of marriage but that she could not have thought that, when put to the test, Charlotte could have married for any reason other than esteem, respect and love. In other word, Elizabeth could not believe that Charlotte could have accepted "so unsuitable a match" just for the sake of a home of her own ("I ask only a comfortable home"):
[Elizabeth] had always felt that Charlotte's opinion of matrimony was not exactly like her own, but she could not have supposed it possible that ... she would have sacrificed every better feeling to worldly advantage.
Thus we see that Charlotte has long held the belief that marriage was a situation in a woman's life that encompassed much more than love and esteem. For Charlotte, marriage encompassed the idea of independence, autonomy and freedom. Her reasons for marrying Collins, then, are exactly what she explains to Elizabeth (excerpt in Question Box).
"I am not romantic, you know. I never was. I ask only a comfortable home; and considering Mr. Collins's character, connections, and situation in life, I am convinced that my chance of happiness with him is as fair as most people can boast ..."
Charlotte, being rational and logical and not a romantic, knows her narrowing options are to remain as a dependent in her father's home or to marry wherever an acceptable opportunity presents itself. While Elizabeth's idea of acceptable opportunity is based upon love and esteem ("every better feeling"), Charlotte's idea of acceptable opportunity is independence from her parents, a home and social life of her own, and a comfortable income and society. In short, Charlotte married Collins because her comfort and advancement in life was of greater importance to her than Mr. Collins' deficiencies were objectionable to her.