Why vinaya forbids monk to ordain physically handicapped? Isn't this a kind of "opposite to empathy, compassion, metta"? 

Nowdays, householders(not all) have cruel behaviour towards handicapped ones. If monk also declines to help, would that be called as compassion?
I do not believe the Vinaya rules prohibiting such ordinations are punitive in nature. They are in place to protect the Sangha jewel. Often monks and nuns need to care for each other in old age as householders will not. As such, many communities will not ordain older or enfeebled people simply because it will place too much of a burden on the healthy monks and nuns and which will result in more suffering not less.

The Vinaya rules may are there to preserve the Sangha jewel which is one of the three hopes of the world. Compassion and metta are foremost in the creation and maintenance of such rules.

Here is an explanation from an Abbey in the West:


  Why does the Abbey have an age limit for ordination? 
  
  The Abbey age guideline is a practical consideration. Many monasteries in Taiwan
  have an age limit, as does Thich Nhat Hanh’s sangha.
  
  In the Vinaya, ordaining preceptors are directed to provide the
  requisites—food, clothing, shelter, and medicine—for their monastic
  followers. Sravasti Abbey commits to lifelong support for monastics
  who ordain here for as long as they live at the Abbey.
  
  At the Buddha’s time, Dr. Jivaka provided free medical care to all
  monastics. There was no health insurance, and if you needed a tooth
  extracted, the barber in the bazaar did it.  Nowadays, medical and
  dental care are costly, insurance premiums are uncertain, and there
  are still out-of-pocket expenses.
  
  Looking long term, we need to be sure that the Abbey can financially
  and emotionally support the monastics we ordain, thus we need a good
  balance of ages in the community. If there are too many elderly
  monastics, it will be a burden on the younger monastics and hiring
  full time care is impossible to afford.
  
  The Abbey is happy to have sangha of all ages visit and train for
  periods of time. The possibility of anagarika ordination remains. We
  also encourage people with monastic aspirations in many other ways,
  including through the annual Exploring Monastic Life program and by
  offering the material in the Monastic Life section of
  thubtenchodron.org.