Victoria wanted new blood in the family (all the European royals are the basically the same family when you go back enough *cough* Anne of Bohemia *cough*), and Louise had fallen for the Marquess of Lorne. So why not let them marry? Victoria's other children and cousins were opposed to a member of the family marrying into the peerage, but Victoria saw no harm in it.
There is no real evidence to support the idea that Victoria really had any interest in bringing 'new blood' into the royal line (not that Louise's descendants would have been royal in any event), save that she used it as an excuse when it suited her. She had a habit of doing that.
Victoria liked Lorne, which was basically all that mattered and once she set her mind on something, it was final. She couldn't accept opposition and family opposition would in many ways simply cement her initial feelings. I sometimes think if the Royal Family had been less hostile to John Brown, she wouldn't have developed such deep feelings for him.
As for Lorne he was Scottish and she loved anything Scottish. I am sure had he been less personally agreeable Victoria would not have allowed the marriage to go ahead on the basis of his non royal status.
She showed no such concern about new blood for example when she forced her grandchildren Ernst of Hesse and Victoria Melita of Edinburgh to marry one another against their wishes. In terms of the British blood line both her heir apparent and his heir apparent (Edward VII and George V) married women who were very closely related to them at Victoria's instigation.
If it were an important thing to Victoria, then Louise would not have been the only one to marry into the British aristocracy.
In fairness to Victoria, although her outlook was Germanic in many ways, she had a very English view on marriage and didn't take any concern in the concept of morganatic marriage, which was something of a German obsession.
For her, it was more important that she approved of the person, which was the case with Lorne and May of Teck, who married her grandson the future George V.
Victoria's opinion of herself was so great that she couldn't understand why others objected, if she approved.