The answer is simple. Alien spouses get only 2-year conditional green cards because this time is considered to be a probation period, during which the couple is supposed to build a record of their married relationship together. USCIS has to make sure that your marriage is bona fide, and that you are not simply trying to become a resident through marriage to a U.S. citizen. In order to evade marriage fraud, USCIS implements all measures to check and determine whether your marriage is legitimate and genuine.
Your conditional status will expire in 2 years. Afterwards, you and your spouse have to file Form I-751, Petition to Remove Conditions on Residence, together with the evidence of a bona fide marriage, proving that you both entered into the marriage in good faith. You have to convince USCIS that the marriage was concluded not for the purpose of evading immigration laws of the USA. Thus, the conditional 2-year period helps to prevent green card fraud.