Why would Jesus command already water baptized disciples wait for the promise of Holy Spirit baptism (Acts 1:5;8) since one baptism(Eph 4:5)?

Mainly, to confound false teachers.

All Christians at the time underwent baptism by immersion in symbolism of the death, burial, and resurrection of Christ.  That is God's commandment for all believers.  That is the one baptism. 

"Baptism" by fire and the Holy Ghost is not and cannot be a real baptism.  The Greek root for "baptize" means to immerse fully or to sink something.  It's time you gave up your gospel hobby horse and took the Bible at face value.

Go back to verse 4 and read through to verse 6 of Ephesians 4. Your calling is unto salvation, and water baptism does not save anyone. The one thief on the cross next to Jesus was promised that he would be in paradise that day, but he was never water baptized. What saved him was his public statement of faith: "Remember me when you come into your kingdom."  Romans 10:9 - "That if thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised him from the dead, thou shalt be saved."  Belief saves, not Baptism. Baptism is something we do to give us the opportunity to confess Him publicly. It is otherwise something we do as a work,,, but our salvation comes from grace, through faith, not by works. Ephesians 2:8,9

All Christians had to be baptised with water Baptism  by Holy Spirit began at christ baptism Acts 10 38 God annointed Christ with Holy Spirit so that Christ could annoint some of the 144000 at Penticost to rule with him in heaven Billions of Christians baptised with water are part of the other sheep Acts 10;16 John the baptiser explains  simply He baptised Jews by full immersion in water to illustrate their repentance Math 3;5-6 Verse 11 one coming after me will baptise with Holy Spirit and fire So there is only One Lord One true God but three baptisms mentioned Christ was baptised by His father after his water baptism He baptised his disciples at Penticost with Holy Spirit  Jesus never baptised any with water and will not be seen by the other sheep again as he also is the same yesterday today and forever in heaven Heb 13;8 Those on earth will see him no more John 14;19 Acts 19;1-7

And when we have a second baptism they are not all the same I have talked with other pastors about this we all had different baptism mine came down from a tree I’m moms yard!

Obviously, (even to you), the PROMISE of John 14-15-16 was EXCLUSIVE to the apostles,(fulfilled in Acts 2:1-4) who were then enabled to preach the first gospel sermon of Acts2, which included the COMMAND to ALL BELIEVERS, v.38, which included the "gift" of the Holy Spirit.  This allows us (even you) to HARMONIZE scripture of Eph. 4:5, (and not force a CONTRADICTION) as it applies to converts.  Thus we have harmony between the PROMISE and the COMMAND.  Sequence and scope, and reliable exegesis and reliable hermeneutics.

Paul wrote Ephesians decades after HS had come and gone.  So when he wrote it, there was only one baptism, water baptism.

Matthew 3:11 provides the answer " I, for my part, baptize you with water because of your repentance, but the one coming after me is stronger than I am, whose sandals I am not worthy to take off. That one will baptize you with holy spirit and with fire."

Because

A

obviously

not "one baptism" in the sense you imagine

B

obviously

because he wanted his disciples to receive Holy Spirit baptism