The first reason is Ukraine’s population.
According to the last population census, on the territory of Ukraine live about 23 millions of women. There are about 19 millions of men for them. If one imagines these numbers percentagewise, it turns out that there are 15% more women than men. The second reason, from women’s words, is a different mentality.
1) Ukrainian men don’t care about their appearance. Most often such a man is associated with a village man. He wears an untidy clothes, is not shaven, smells of sweat, his head is dirty tec. Western men in their majority (according to a romantic image of Ukrainian women), on the opposite, take much care of themselves. They are clean shaven, wear tidy fashionable clothes, smell of expensive perfumes.
2) Ukrainian men forgot how to court. Maximum what a Ukrainian man is able is to do is to invite a girl to the cinema or a restaurant. They give flowers only on holidays. Foreigners (especially during the first months of dating) give flowers always and everywhere – they order them to their room, buy around a corner, pick them from flower-beds violating laws.
3) Compliments. Personal messages from social networks will tell better about it. The first fragment from correspondence with a guy from Russia. Without preludes and introductions: ‘I would like to stay at your place.’ The second quotation is from a correspondence with a man from Cuba: ‘I send you a positive energy, the energy of the sun which you will feel warmth from. I send you feelings of my heart to warm you when it is cold.’

The third reason is a romantic image of a foreign country.
There is a saying that it’s good but not here. Many people (not only women) want to change their life. The simplest variant is to change one’s place of living. It is very difficult to go abroad to work, but it is more than pleasant to go on an invitation of a handsome man from a dating site. One should note that some girls live with foreign men just because they don’t want to come back to Ukraine. To live abroad, no matter where exactly, is much more prestigious than to live in a home city in Ukraine. One can always boast for classmates to see in social networks.
One should note that it’s just a superficial opinion. A detailed analyses of reasons making brides of Ukraine look for their happiness abroad can be easily found on the site ukrainianbrides.ru. However, there are as many opinions as there are people.