Because the one who prays is whispering to his Lord, and it is obligatory for the one who whispers with his Lord to keep his heart free for whispering to his Creator and Master, and not to occupy his heart with any worldly matters that would distract him from whispering with his Lord.

Sayyiduna Abu Hurairah (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) led us in the Zuhr prayer. When he finished the prayer with salam, he called out to a person who was at the end of the rows and said: "O so-and-so, do you not fear Allah? Do you not reflect on how you have performed the prayer? Indeed, when any one of you stands for prayer, he stands only to converse privately with his Lord. Therefore, he should consider how he is conversing with his Lord. Do you think that I do not see you? By Allah, indeed I see you from behind my back just as I see you in front of me."