Why Priests are not allowed to marry in Roman Catholic Church?

Now concerning the things whereof ye wrote unto me: It is good for a man not to touch a woman.
1Co 7:2 Nevertheless, to avoid fornication, let every man have his own wife, and let every woman have her own husband.
1Co 7:3 Let the husband render unto the wife due benevolence: and likewise also the wife unto the husband.
1Co 7:4 The wife hath not power of her own body, but the husband: and likewise also the husband hath not power of his own body, but the wife.
1Co 7:5 Defraud ye not one the other, except it be with consent for a time, that ye may give yourselves to fasting and prayer; and come together again, that Satan tempt you not for your incontinency.
1Co 7:6 But I speak this by permission, and not of commandment.
1Co 7:7 For I would that all men were even as I myself. But every man hath his proper gift of God, one after this manner, and another after that.
1Co 7:8 I say therefore to the unmarried and widows, It is good for them if they abide even as I.
1Co 7:9 But if they cannot contain, let them marry: for it is better to marry than to burn. 1 Corinthians 7:1-9

Matthew 19:12 For some are eunuchs because they were born that way; others were made that way by men; and others have renounced marriage[ 19:12 Or have made themselves eunuchs] because of the kingdom of heaven. The one who can accept this should accept it.

1Now for the matters you wrote about: It is good for a man not to marry. But since there is so much immorality, each man should have his own wife, and each woman her own husband. The husband should fulfill his marital duty to his wife, and likewise the wife to her husband. The wife's body does not belong to her alone but also to her husband. In the same way, the husband's body does not belong to him alone but also to his wife. Do not deprive each other except by mutual consent and for a time, so that you may devote yourselves to prayer. Then come together again so that Satan will not tempt you because of your lack of self-control. I say this as a concession, not as a command. I wish that all men were as I am. But each man has his own gift from God; one has this gift, another has that. Now to the unmarried and the widows I say: It is good for them to stay unmarried, as I am. But if they cannot control themselves, they should marry, for it is better to marry than to burn with passion. (1 Cor 7:1-9)

1Here is a trustworthy saying: If anyone sets his heart on being an overseer,[1] he desires a noble task. 2Now the overseer must be above reproach, the husband of but one wife, temperate, self-controlled, respectable, hospitable, able to teach, 3not given to drunkenness, not violent but gentle, not quarrelsome, not a lover of money. 4He must manage his own family well and see that his children obey him with proper respect.

8Deacons, likewise, are to be men worthy of respect, sincere, not indulging in much wine, and not pursuing dishonest gain. 9They must keep hold of the deep truths of the faith with a clear conscience. 10They must first be tested; and then if there is nothing against them, let them serve as deacons.