If there are no restrictions in Buddhism then why do they not want to get married?
Of course there are restrictions!
Abrahmacariya veramani sikkhapadam samadiyami (I will refrain from sexual activity)
If you can't have sex, your spouse probably won't be too pleased. Having a family of your own will also prove slightly more difficult. Besides, the whole point of forgoing the householder life for homelessness is to pursue enlightenment with the minimal amount of obstacles possible. Supporting a family is quite a large responsibility to undertake!
(Note: this does not apply to all Buddhist schools. Zen monks often marry. But if you are looking for the strict, scholastic answer, see above.)
Some do get married! It just depends on the tradition.
In the 2005 article "I married a monk" the excerpt below speaks of some notable examples
Finally, the master spoke in a sermon-like tone, looking off into the distance. “There are precedents and justifications for marriage in Buddhism,” he said. “A famous monk, Manhae, was known for his love poems, which many say could not have been written had he not loved a woman deeply. Then there is the seventh-century monk Wonhyo, the most famous monk in Korean Buddhism. It’s said that the king asked him to sleep with his daughter, the princess, to console her. Wonhyo saw this as skillful means. In the Tripitaka, there are examples of the Buddha in dialogue with householders. He teaches how householders can live a pious life.”
If you read the entire article you will find the master struggled to come to this view, and at first was opposed to the marriage. A typical battle of tradition vs modern life.
This case was a Korean Zen monk but other traditions have a place for marriage for those that want it.
In Tibetian Buddhism Monks and lamas don't necessarily have to be celibate, and sometimes get married.
In Japan monks and nuns can marry as well.
Different Traditions and vow sets have different rules. I believe that the other traditions all have a vow of celibacy, but please correct me if I am wrong.
One interesting thing to note, for those who do take the vows, is that sexual misconduct for monks and nuns holding the vows includes masturbation.