Who said they didn't or did?! Who got that ultimate siddhi/special vision and power to declare that they got mOksha?! Neither Vyasa got into any of these nor he is one person (It's a post)! Also, Bhagavad Geeta is completely misunderstood and misquoted most often and here too. Krishna said "come to ME". But Who is this ME?! ME is not that human Krishna who lived during Dwapara yuga. It's aatman, Brahman, ultimate Ishwara, our very own SELF!
Most people use the word "mOksha" without knowing anything about what it means. mOksha is just knowing that you are the divine yourself. But it's not simple, rather far complex when it comes to removing the ignorance permanently that God/Lord is somewhere sitting in Vaikunta or Kailasa!
So, Bhishma definitely is the candidate considering the level of "purity of mind" he achieved and demonstrated throughout, unparalleled determination and ultimate saadhana / efforts he put in for understanding and achieving "mOksha" (Jnaana yOga). We see everyone including Krishna suffered during their death, but Bheeshma overstayed 48 days more with extreme effort to avoid rebirth (study the Akshara Brahma yOga of Bhagavad Geeta). Other 2 were great candidates but we don't have clear picture of their status of mind.
MOST Indians are lost in tons of devotional / mythological literatures (more of myths and legends symbolizing something) created during the economic downturns due to invasions / slavery etc. Simply following blind faith without shraddha (inquiry followed by faith) or efforts to understand mythology by decoding the hidden code, questioning and analyzing the purpose of these great epics and the science behind! We're so lost in the "avataara" fairy tale and thus can't comprehend the science of vEda / VEdanta. Purpose of creating Bhagavad Geeta was to let people understand the "Advaita" (everything in one)! Nothing to do with the worship of the Krishna as a person born once (though it got exploited and turned to wrong side)!
On this same note, gosh so much time gone in defending everything Pandavas did or Krishna did and other way around to thrash those in against them (Vyasa is guidance of how to read/understand Mahabharata is long gone!)!
Hope this clarifies. Feel free to raise questions!