It has armor that is almost as thick as the Centurion which definitely is considered a main battle tank, it has a powerful gun and despite its weight being in the category of a heavy tank, it has a very high power-to-weight ratio which makes it just as mobile as any medium tank that was in service during WW2.
Though it was unreliable especially early on and somewhat overengineered, it has all the desirable qualities of a main battle tank yet it is not considered one. Why?
Note that the Centurion wasn't actually designed as a 'universal' or main battle tank, but as another Cruiser tank. The only reason it's often considered to be be one of the first true MBT is that it's upgraded (its was significantly upgunned a few years after the end of WW2) specs have all the desired properties of one. The distinction of what constitutes a MBT isn't set in stone and whether a tank qualifies is of course also dependent on how it compares in its own time period; e.g. for 1980 standards, neither the Panther nor the Centurion have good armour or firepower, and would be considered slow.
"So why isn't the Panther really considered the first MBT?" Perhaps the best argument against considering it a MBT, or a forerunner of the MBT is that post-WW2 tank designs took relatively little from the Panther design, and wasn't the 'top tank' to replace all others, in contrast to the Pershing and the Centurion, which both pioneered properties that would keep on being used. I doubt anyone would really object if you called it a proto-MBT or something like that though, because the concept of a 'universal tank' had been around even long before the first drafts of the Panther. The Panther was fast, capable of penetrating pretty much any other tank and was well-armoured, but what drove the 'Main Battle Tank' distinction is rather that -whereas the Panther still had 'bigger, heavier brothers' like the Tiger- the overall increase in penetrative capability of Cold War guns, the improvements in engine technology and development of composite armour led to a whole shift in design doctrine and ended the feasibility of (super-)heavy tanks pretty much completely.
So in conclusion: the concept of the Main Battle Tank had been around for a while, but only became a practical reality some years after WW2, when it essentially replaced all other design doctrines. While an overall superb design (if we ignore those pesky reliability issues), the Panther was not the 'tank to replace all other tanks', because it could not fill all battlefield roles; whereas the upgunned Centurion and later Cold War designs did.
While the 75mm L/70 is powerful, it was still a limited armament chosen due to size constraints and very much sacrificed cannon performance. While against mediums this was less noticeable, the Panther would be hard pressed to deal with later heavy tanks at longer ranges and even at short ranges against heavy tank projects at the end of the war.
In particular, in just the German arsenal, the 75mmL/70 comes up short to the long 88, the German choice of armament for their later heavy tanks. It again comes up short against the 122mm cannon the Russians used for long range anti-tank combat, or against the 105mm or 120mm cannons of choice for American heavy tank projects. The Panther was ill-suited for any such up gunning to put it on even footing armament and thus ability to kill heavy tanks at range. One of the important elements of the MBT is that it packs the firepower of what would be a heavy tank while keeping the general purpose ability of a medium tank.
You also might as well ask why isn't a tank like the T-34 considered a main-battle tank despite the fact that early on, they considered the T-34 as being capable of performing the same roles as the KV-1 ?