Because unfortunately, religion is frequently used as a means of oppression. Yoga is a form of freedom.

Strangely enough Yoga and Islam have soo much in common , infact if anything Yoga has not only helped me in my spiritual path but also my path to God , if you look at salat (muslim prayer done 5 times a day) What is it ? Worship, meditation, and yoga postures

I have heard the same reasons for not trying Yoga from people of Christian faith. The thought of sitting in prayer position, in deep meditation makes some people uncomfortable. "Who are you praying to?" is a question I have heard before. It seems that the negative feedback are from people that are looking at it superficially, not from the internal benefits one can receive. Yoga is within religions, but religion is not within Yoga. It is only when you mix the two does Yoga appear to be a religion.

Because little by little yoga produce an inner peace that put you in a naturally meditative state and from that you realize that there are other ways to approach divinity that the rituals of your traditional faith. All priests of whatever religion know that by instinct.

A regular yoga practice strengthens a person's faith, it doesn't weaken it. Any person of any religion can practice yoga and know that their belief systems will only deepen. The practioner will only experience a deepening sense of spirituality.

Religious leaders by definition tell people what to think, Anything that proomotes the idea that humans have the answers they need within themselves will be suspected by them at best and outlawed at worst. I think this is why China outlawed Falon Gong.

1) Fatawa councils within Islamic context are merely interpreting the teachings of Quran and tradition of Prophet Mohamed. One such tradition states that 'we should abstain from doubtful acts/activities/actions etc.' So this particular Fatwa (Note: not legally binding) is merely highlighting the fact that Muslim should make a conscious effort to know what they are doing is appropriate or not when it come to Yoga. As another tradition of Prophet states that 'Action are based on intentions' so if one is practicing Yoga to attain health benefits then it is OK to do so but if one is practicing yoga as a form of religious activity then there is something not right.
2) Yoga instructions are sometime sublimely attached to Hindu religious teaching. There is a need for clear distinction between what is a-spiritual and spiritual aspect of yoga. I guess knowing that distinction will help people make better informed decision.
3) The 5 daily daily prayer in Islam is an example of meditative exercise so yoga meditation is not necessary in Islamic context.