I guess he has denied because the one thing USADA don't have is the smoking gun, the failed drugs test.
A lot of people can't get their head around the fact that most other sports catch their cheats with drugs tests, and this didn't happen in Cycling. This overlooks that EPO (the drug of choice for cyclists) didn't have a test for a long time, and even when it did the athletes didn't 'glow' for long after taking it so it was easy to avoid without effective out of competition testing. Which there wasn't.
Even now there is not a foolproof test for the other 'drug' of choice which is Transfusions. Hence the need to have a Blood passport which only shows up anomalies over time.
Armstrong’s suspicious test for EPO at the 2001 Tour of Switzerland
The 2001 Tour du Suisse (Tour of Switzerland) was conducted from June 19 – 28, 2001 and was won by Lance Armstrong. Armstrong told both Tyler Hamilton and Floyd Landis that he had tested positive for EPO at the 2001 Tour of Switzerland and stated or implied that he had been able to make the EPO test result go away.
Armstrong’s conversation with Hamilton was in 2001, and he told Hamilton “his people had been in touch with UCI, they were going to have a meeting and everything was going to be ok.”
Armstrong’s conversation with Landis was in 2002, and Landis recalled Armstrong saying that, “he and Mr. Bruyneel flew to the UCIheadquarters and made a financial agreement to keep the positive test hidden.”
Consistent with the testimony of both Mr. Hamilton and Mr. Landis, Pat McQuaid, the current president of UCI, has acknowledged that during 2002, Lance Armstrong and Johan Bruyneel visited the UCI headquarters in Aigle in May 2002 and offered at least $100,000 to help the development of cycling.
UCI vehemently denies that this meeting or payment was, as Mr. Armstrong told Mr.Hamilton and Mr. Landis, tied to a cover-up of the 2001 Tour de Suisse sample. In any case,what is important for the case is that substantial parts of Mr. Hamilton’s and Mr. Landis’s recollections of Mr. Armstrong’s statements have been corroborated.
As discussed in more detail in Section V(C) below, Dr. Martial Saugy, the Director of theWADA-accredited anti-doping laboratory in Lausanne, Switzerland has confirmed to both USADA and the media that his laboratory detected a number of samples in the 2001 Tour deSuisse that were suspicious for the presence of EPO. Dr. Saugy also told USADA that he was advised by UCI that at least one of these samples belonged to Mr. Armstrong. Therefore, even without any consideration of the laboratory test results for these samples, as set forth above,Tyler Hamilton’s and Floyd Landis’s testimony regarding Mr. Armstrong’s admission that heused EPO at the 2001 Tour of Switzerland finds substantial corroboration in the statements of both Dr. Martial Saugy and UCI President Pat McQuaid.