Can anyone tell me why P.Floyd is prog?

What is it about Pink Floyd that makes them prog?

prog or not, they are the best

If you listened to Dark Side or the later albums starting with The Wall, your description is correct. But if you listen to stuff from Piper to Obscured By Clouds, and Wish You/Animals, you will see long compositions, complex arrangements, and all other characteristics of prog perfectly present. They had long, complex epics like Atom Heart Mother, Echoes, Saucerful Of Secrets, Alan's  Psychedelic Breakfast and Shine On You Crazy Diamond. Animals is a very "prog", extremely complex concept album. They even experimented with avant garde on Ummagumma. Are you sure you heard the right albums? 

Let's look at the PA definition of prog.

• Long compositions, sometimes running over 20 minutes, with intricate melodies and harmonies that require repeated listening to grasp. These are often described as epics and are the genre's clearest nod to classical music. An early example is the 23-minute "Echoes" by Pink Floyd. Other famous examples include Jethro Tull's "Thick as a Brick" (43 minutes), Yes' "Close to the Edge" (18 minutes) and Genesis' "Supper's Ready" (23 minutes). More recent extreme examples are the 60-minute "Light of Day, Day of Darkness" by Green Carnation and "Garden of Dreams" by The Flower Kings.

I mentioned the epics above.

• Lyrics that convey intricate and sometimes impenetrable narratives, covering such themes as science fiction, fantasy, history, religion, war, love, and madness.

Pink Floyd were one of the first to experiment with narratives, Barrett and later Waters were great lyricists who covered all these aspects.

• Concept albums, in which a theme or storyline is explored throughout an entire album in a manner similar to a film or a play. In the days of vinyl, these were usually two-record sets with strikingly designed gatefold sleeves. Famous examples include The Lamb Lies Down on Broadway by Genesis, Tales from Topographic Oceans by Yes, 2112 by Rush, Dark Side of the Moon and The Wall by Pink Floyd, and the more recent Metropolis Part II: Scenes from a Memory by Dream Theater and Snow by Spock's Beard. Aqualung, perhaps the best-known record by Jethro Tull, is often regarded as a concept album due to its recurring themes, but songwriter Ian Anderson has always claimed that the album is just "a bunch of songs".

This point well covered too. Although I think Animals beats these two.

• Unusual vocal styles and use of multi-part vocal harmonies. See Magma, Robert Wyatt, and Gentle Giant.

Plenty, especially on Animals and The Wall.

• Prominent use of electronic instrumentation — particularly keyboard instruments such as the organ, piano, Mellotron, and Moog synthesizer, in addition to the usual rock combination of electric guitar, bass and drums.

They used all those instruments, except I'm not sure about Moog and Mellotron. 

• Use of unusual time signatures, scales, or tunings. Many pieces use multiple time signatures and/or tempi, sometimes concurrently. Solo passages for virtually every instrument, designed to showcase the virtuosity of the player. This is the sort of thing that contributed to the fame of such performers as keyboardist Rick Wakeman and drummer Neil Peart.

PF did amazing drum perfomances, weird guitar and keyboard work on the early albums(watch Live at Pompeii), later it was reduced to more straightfoward, but still as technical and virtuose solos.

• Inclusion of classical pieces on albums. For example, Yes start their concerts with a taped extract of Stravinsky's Firebird suite, and Emerson Lake and Palmer have performed arrangements of pieces by Copland, Bartók, Moussorgsky, Prokofiev, Janacek, Alberto Ginastera, and often feature quotes from J. S. Bach in lead breaks. Jethro Tull recorded a famous cover of J. S. Bach's "Bouree", in which they turned the classical piece into a "sleazy jazzy night-club song", according to Ian Anderson. Marillion started concerts with Rossini's La Gazza Ladra (The Thieving Magpie). Symphony X has included parts by, or inspired by, Beethoven, Holst and Mozart.

They didn't have that one, although Atom Heart Mother is a classical piece of its own.

• An aesthetic linking the music with visual art, a trend started by The Beatles with Sgt. Pepper's and enthusiastically embraced during the prog heyday. Some bands became as well-known for the art direction of their albums as for their sound, with the "look" integrated into the band's overall musical identity. This led to fame for particular artists and design studios, most notably Roger Dean, whose paintings and logo design for Yes are so essential to the band's identity they could be said to serve the same function as corporate branding. Hipgnosis became equally famous for their unusual sleeves for Pink Floyd, often featuring experimental photography quite innovative for the time (two men shaking hands, one of whom is in flames, on the cover of Wish You Were Here). H.R. Giger's painting for Emerson Lake and Palmer's Brain Salad Surgery is one of the most famous album sleeves ever produced.

The most amazing works of Storm are PF sleeves.

All points except for one covered. Need any more proof?

Let's drop the notions of long complex arrangements, complexity and all the other nonsense that tends to go with describing prog for a moment.

Prog Rock is not about being hideously complex, or being able to improvise for hours on end -although Pink Floyd more than demonstrated their abilities on their earliest output. In fact, if you think about it, everything after Meddle was one concept per album - the Wall being the ultimate consumation of coherent concept-building.

The biggest problem with Pink Floyd as a Prog Rock band that I see is modern perception of Prog Rock - which seems to have little or nothing to do with actual Prog Rock, and more to do with casual sources of reference like Wikipedia, which is one of the worst sources of information about music that I have ever seen - simply because of all the opinionated and factually inaccurate articles therein.

Prog Rock is defined by the bands who were there first - the bands who defined Prog Rock.

Pink Floyd are one of those bands.

That's why they're Prog