Most English person pronouns have an objective case — I/me, we/us, thou/thee, he/him, she/her, they/them, who/whom. But "you" and "it" have no such form. Did they every have one? is there any reason for this?
it is not too surprising, but the collapse of thou/thee and ye/you into you is notable and rare among the languages of the wider Western world at least.
Fairly suddenly in the 17th century, thou began to decline in the standard language (that is, particularly in and around London), often regarded as impolite or ambiguous in terms of politeness. It persisted, sometimes in an altered form, particularly in regional dialects of England and Scotland farther from London, as well as in the language of such religious groups as the Society of Friends. Reasons commonly maintained by modern linguists as to the decline of thou in the 17th century include the increasing identification of you with "polite society" and the uncertainty of using thou for inferiors versus you for superiors (with you being the safer default) amidst the rise of a new middle class.
See also: Middle English creole hypothesis.
I would not say that these pronouns lack an objective case. It is just that the subject (nominative) and object (accusative) forms are identical. In Old English, as in virtually all Indo-European languages, neuter nouns and pronouns always have the same form in the nominative and accusative, in the case of Old English "hit" for the 3rd person singular neuter. This is striking archaic feature in modern English.
I would say that whatever reasons there are are historical. If you look at related Germanic languages, they all have the same form for the neuter singular pronoun in the nominative and accusative (Dutch het, German es, Norwegian det, Icelandic það, for example), but all them of them that I can think of distinguish the two cases in the second person singular (Dutch jij/jou, German du/dich, Norwegian du/deg, Icelandic þú/þig). When one form is used for two functions, as in the neuter singular it is called syncretism; the nom/acc syncretism for neuter singular is an old Indo-European trait. Apart from the neuter singular, there is some variation even within the Germanic languages whether a language distinguishes nominative and accusative or whether a single form is syncretic for both. Icelandic distinguishes nom and acc in second person plural (þið vs. ykkur) and third person masculine plural (þeir vs. þá), but has nom/acc syncretism in the masculine singular (hann) and in the feminine plural (þær). German has nom/acc syncretism in the feminine singular (sie) and the third person plural (also sie), but distinguishes them in the masculine singular (er vs. ihn).