If you mean why he was not written into Caesar's will, the answer to that is simply that legally he couldn't. Caesarion was neither a citizen nor a legitimate child, nor could he ever have been, as Cleopatra was a non-citizen. By Roman law non-citizens could only be named in wills if they were the slaves of the testator, and only if he freed them in the same will, in which case they became instantly Roman citizens. In Caesar's lifetime Roman citizens of the first property class could also not name women as their heirs by the lex Voconia, although that of course would not have mattered since even if Caesar wanted to include Caesarion in his will through Cleopatra she was of course not a citizen.

In any case Caesar was not about to give Caesarion anything. Whether Caesarion really was Caesar's biological son his parentage was not an important question until after his death. Caesarion is not mentioned by any text prior to Caesar's death, particularly not by Cicero, who met Cleopatra and mentions her in his letters (he found her disgustingly arrogant). Antony's claim that Caesar had openly acknowledged that he was Caesarion's father was legally a non-issue, as Caesarion could neither inherit Caesar's name nor his fortune. Rather it was a way of calling into question the validity of Octavian's claim to sole heir of everything Caesarian, and provoked a pamphlet from Oppius attempting to prove that Caesarion could not have been Caesar's son. Caesarion was about a year old when Caesar's will was written, and he was only three years old in March of 44. Even if Caesar had wanted to grant Caesarion some sort of unofficial inheritance it's simply not plausible for him to have gotten anything at the tender young age of three. Besides that, surely Caesar understood the serious problems that would arise from having an illegitimate foreign toddler inherit. A generations later Octavian was able to rouse the hatred of Italy by suggesting that Cleopatra, the foreign queen, had completely ensnared Antony and was bent on seeing her ambitions brought into the city itself--Cleopatra was already seen by Cicero and others as rather distasteful, and it's pretty hard to believe that even if he legally had been able to give Caesarion anything he would have.

Of course, Caesar did not, nor could he legally. According to Suetonius Caesar gave Cleopatra and Caesarion many gifts and allowed Cleopatra to use his name. If this really happened and isn't just based on Antony's rhetoric and Cleopatra's unsupported claims then that's all that Caesarion ever received from Caesar and was ever likely to receive, given his status