ULTRA didn't necessarily provide omniscience. There are plenty of times when access to ULTRA at the tactical-level, in a timely manner, still did not translate to Victory. The Allied failure to win at Crete despite the disastrous German opening is a prime example of a potential failing of ULTRA intercepts; interpreting them.

Operational Security means ULTRA intercepts cannot be highly detailed, and at Crete Freyburg famously misinterpreted the implications of the intelligence he was given; denuding rather than reinforcing airfields' defenses. Never discount commander error.

Further, ULTRA wasn't foolproof; it was based on radio intercepts and the reality is that communications at the operational level often occurred over telephone wires for the Germans. A combination of commander errors on the Allied side, operational security on the Axis side, and the increasing lack of need for radio communications could mean that certain Axis plans were not divined fully to the Allies. The most famous example being the Bulge offensive; which was a heady combination of Allied under-estimation of remaining mobile German strength and a rare security success for German planning; often attributed in part due to the lack of radio communications, and therefore ULTRA intercepts.

Finally...

However, it seems that the Allies had pretty broad access to the decrypted Ultra messages by the time of the Market Garden initiative. Why then did it fail?

The immediate, Day One and Day Two counteractions by the Axis to the three vertical envelopments were absolutely adhoc, and the order of battles in the actions against the three Airborne divisions are an undeniable mess. We have Kriegsmarine coast watcher and rifle units, Security units, NCO Cadet schools, Replacement Brigades and other second line formations being cobbled together into battlegroups to attempt to give the German commanders on scene the flexibility and manpower to react as immediately as possible. To an extent, this worked; the 101st took its objectives and prevented the retreat of the forward most units, but lost key bridge crossings and suffered heavily. The 82nd was stymied in taking Nijigmen completely until the XXX Corps arrived, and the 1st Airborne was immediately and hotly engaged for Arnhem upon landing. The reactions were expected, but not in such force. As mentioned, a large part of why it was not expected is because the counter-movements were adhoc and made with a minimal of communication, and the Axis radio network in occupied Holland was far from extensive to begin with, having left most communication and other heavier equipment behind in their disastrous retreat through France. ULTRA failed because it never had a chance to succeed in these circumstances, and the gaps in allied intelligence viz. the German's combat effectiveness in Holland were already based on shaky foundations and understandable confidence to begin with.