God told us to pray 5 times each day. Why? We do not know it. I suppose to never lose contact with Him.
Before, He taught the Jews to pray 3 times each day and keep one entire day to Him. Supposedly, this is also sufficient to stay in contact with Him.
Why did He alter the recommended prayers? We don't know it. Supposedly, because it is all to make us stay in His spirit all the time and in what we do in our daily lives.
How will He judge? We do not know it exactly. But He said that He will judge on our good deeds that will result from our faith and on the transgressions that result from our negligence.
God knows better
To complete the answer:
I have read that the Prophet (p.b.u.h) has chosen the time for the juma prayer because it was after the market when most people, also those who did not live in the town of Medina, could assemble. Me, too, as i live 50 min (each way) from our Islamic center, I can not go for common prayer on normal days but I try to be there for Juma whenever I can. The Juma is established as the common prayer, and if you have difficulties to attend daily prayers in the mosque it is the occasion to go and meet for common prayer. So, yes, Juma is the most important prayer.
I did give a reason why somebody who does not do daily prayer may go astray but that there is finally only One who will judge.