Well the first thing we have to examine here is that although they may be superficially equivalent in theory, SEATO actually had some key practical differences that really meant that comparing the two has challenges associated with it.
For a start, NATO was a mechanism used by senior partners as a bulkwark against the USSR and as such was well resourced and committed to from a practical military perspective pretty much from the get-go.
Recently deceased Australian military historian Jeffrey Grey almost paints SEATO as the opposite - a mechanism used by junior partners to compel senior partners in a similar venture to act as a bulwark against communist nations and interests by exploiting the political will of some senior ex-colonial partners to remain in Asia (such as the UK and France) or outright trying to rope the US into acting as a security guarantor (in the case of the US) in that capacity for the benefit of the alliance's junior partners such as Australia, New Zealand, Thailand, Pakistan, the Philippines, (the Kingdom of) Laos, Cambodia and the then Republic of Vietnam. In that situation that critical commitment is less likely to be (and was proven to not be) forthcoming.
This is possibly a little cynical on Grey's part, considering John Dulles' involvement with creating SEATO, but his cynicism is understandable, given his focus on the amount of concern raised in the US Congress about it (the Senate vote was notably in favour of it) and that smaller nations with smaller defence budgets arguably had more to gain from having others underwrite their security than the US, Britain and France had in having local junior partners to help implement policy (especially since more than one of those member states were dealing with communist insurgencies during the period SEATO was active).
Almost immediately after the creation of SEATO Headquarters in Bangkok in 1956, the US made it clear that no formal military commitment to that command would be forthcoming. This not only hindered operational viability, but it also constrained planning for obvious reasons (as you can't plan with military assets you don't have) by that headquarters.
Countries like Australia and New Zealand were already, quite separately from SEATO, contributing to a British Commonwealth strategic reserve, but this was primarily for forward defence of Commonwealth and Imperial interests more generally, with providing a counter to communism as an afterthought; Grey notes that Australian prime minister Robert Menzies use of SEATO as a pre-text for deploying Australian forces to what was then British Malaya in the mid-1950's didn't really qualify as a SEATO commitment because Malaya was not a sovereign nation at the time (and Malaysia once formed was not a SEATO member in any case, which was why the Strategic Reserve was not formally used in Malaysia in the mid 1960's, even if member state forces did provide commitments in Malaysia at that time).
So from the above examples we can also see that SEATO was, far from being a cornerstone of a strategic imperative, not really taken seriously even by the nations that may benefit most from it - it was not invoked as a treaty organisation and used to intervene either in Laos or in Vietnam, with intervention in both countries occurring unilaterally by various parties.
This lack of meaningful commitment would ultimately translate into practical withdrawal as both the UK and France scaled back their commitments in South East Asia in the mid 1970's as their post-colonial interests there shrank. That contributed to Australia and New Zealand's own review of their strategic postures on forward defence, especially in light of budgetary pressures.
SEATO essentially became a club no one was interested in being a part of, and only some members were only interested in it on a cursory basis in the first place, if that.
SOURCES:
Grey, Jeffery - 2008 - A Military History of Australia (3rd Edition)
Blaxland, John - 2006 - Strategic Cousins
First, the US still considers the mutual defense aspect of the treaty alive. In addition to other US treaties, this only additionally binds the US to Thailand in regards to defense, but also includes the UK, Australia, New Zealand, France, and the Philippines.
It dissolved because it was ineffective. Its primary goal throughout its existence was to protect South Vietnam and Laos (who weren't allowed to join the alliance) from Communist expansion, but failed in both regards. With those failures, the withdrawal of Pakistan due to the independence of Bangladesh, and general disinterest among many of the members, SEATO agreed to dissolve itself because there wasn't really a reason for it to exist. The states of former Indochina had already fallen to communism. The US and UK took active roles suppressing communist insurgencies in Indonesia, Malaysia, and the Philippines without the need to appeal to a larger treaty organization. Thailand was still given security guarantees by the US. Also, with Nixon going to China, there was less threat of an overarching Communist bloc forming in East Asia as the Sino Soviet split was solidified. This eased Western concerns regarding further communist expansion in SE Asia.
In general, it didn't last longer because it failed at what it was supposed to do and internal dissent broke it apart.
Edit: Just noticed the extra word in the title. Sorry, too late now!