First of all, they were Americans. Everyone knew . Many, like the Duchess of Marlborough exclaims that Lady Blanford says, "thought we all lives on plantations with negro slaves and that there were Red Indians ready to scalp us just around the corner." (Balsan, pg. 55) It was quite common for Americans abroad to be told that "no one would take you for an American," (Balsan, pg. 76) this being meant as a very high compliment. The wariness, however, was mutual: it had been no secret which side the textile-producing British had supported during the Civil War, and it took a while before many Americans were willing to forgive and forget.
Secondly, when you got down to it, these American heiresses were no different from the cadre of upstarts at were hanging around London: why on earth did they need to import more?
Eventually, most of these women were accepted into polite Society, which led directly
Along, however, came the Prince of Wales. Prince Albert (later Edward VII) loved nothing more than food, parties and beautiful Americans. All that an heiress had to do was to catch his eye, and she would be "in." The Prince of Wales was even the deciding factor in many of these marriages: it was he who convinced Lord Randolph's father, the Duke of Marlborough, to allow his marriage with Jennie Jerome. Many of these girls, including probably Lady Randolph Churchill, later became his mistresses (although all three of his official mistresses were of British birth), but for now he just liked to surround his ever-widening girth with young American beauties.
There were, however, only so many heiresses, and they had only so much money (although they brought with them as much as two million pounds), and that really had to be used for practical matters, such as installing running water, patching up roofs, and paying off debts. Society wanted some fun and they wanted it immediately.