While the short answer is simply that they didn't want to, the truth is that in early 1917 nobody in the Soviet movement was thinking about unilaterally seizing power.

It's important to understand that, contrary to the role they would later play, the soviets had never been intended as organs of administrative rule or legislative chambers. They had developed in 1905 as essentially corporate bodies, formed to represent the interests of workers in negotiations with the state. Unlike the Duma, from which the Provisional Government sprung, there was nothing in the history or form of the soviets to suggest that they might become the basis of a new state.

The same can be said of the thinking of the socialist intelligentsia. With the arguable exception of Trotsky's Mezhraiontsy, no faction of the RSDLP had attached any real importance to the revolutionary potential of the soviets after 1905. Both Bolsheviks and Mensheviks accepted that the coming revolution would be a bourgeois-liberal one and that only the completion of this would permit a second, socialist, revolution. Ironically, Lenin was the more assertive here, arguing prior to 1917 that socialists should participate in a provisional 'revolutionary-democratic dictatorship'; the Mensheviks believed that workers' bodies should only exert pressure from outside the government.

And it was the Mensheviks that got their way in early 1917: they, along with the Socialist Revolutionaries, dominated the Petrograd Soviet, the First All-Russian Congress of Soviets and their respective executive committees. The soviets turned out to be the perfect vehicle for the workers to supervise/control the Provisional Government without sullying their hands with joining a 'bourgeois' government. (Initially at least - reservations of 'Millerandism' were put aside after the April Crisis.) This is well captured in a Menshevik paper editorial from March 1917 that's worth quoting at length:

In its essence, our revolution is bourgeois-democratic. . . . Given the country’ s historical and economic conditions, it cannot be otherwise. Our capitalists are still too weak and our productive forces are still too underdeveloped... [The armed people] cannot begin to press for a socialist revolution. That would lead only to anarchy. The revolutionary masses knew this instinctively. The proletarian vanguard—the majority of Social Democrats, clearly understood this.

To its great historical merit, the Russian proletariat consciously supported the bourgeois democracy's struggle for power and supports the creation and consolidation of a new democratic order. This support will be steadfast and unwavering as long as the bourgeois democracy, in the form of the Provisional Government, strictly fulfils the revolutionary obligations that it has taken upon itself.

The Provisional Government must be given full power. Only then can the country's economic life become organized. Only then can its great economic problems be solved. Only then can urgent political and general social reforms be implemented...

The Soviet of Workers' Deputies must openly and fully support the Provisional Government, as long as it honestly makes necessary reforms and firmly upholds the banner of the Constituent Assembly.

On display here are the key themes of the Menshevik view of the February Revolution: a bourgeois revolution against the Autocracy that must be conditionally supported by the proletariat. Only when this was complete could the progression to socialism take place.

This was all very orthodox Marxism and uncontroversial at the time. No one had anticipated a socialist revolution in Russia of all places. Even the Bolshevik leaders in Russia were amenable to supporting the Provisional Government. It wasn't until Lenin arrived back in April 1917 that the cat was set amongst the pigeons. His April Theses were absolutely radical: the Menshevik Sukhanov recalls how Lenin's speech on 03 April (Old Style) "burst on us something … novel, harsh and stunning". His calls for an immediate transfer of power to the soviets shocked his audience and led to accusations of "semi-anarchism". Yet he was soon able to win his party over (supporting Robert Service's suggestion that the Bolshevik rank and file were keen for radical action) and take it into outright opposition.

'All power to the soviets' soon become a very potent Bolshevik slogan. It presented in a nutshell the call for a class-based solution to the war and crises. As early as 18 June (OS) the Soviet leadership was sharply embarrassed when a mass demonstration they called was dominated by such Bolshevik slogans calling for a socialist government. From early summer this intransigent Bolshevik position began to gain popularity with the soviet electorate. This progressed alongside the rapid polarisation of the country, particularly after the Kornilov Affair. At this point Sukhanov, reflecting on the balance of power on the streets and factories in August 1917, accepted that:

The [Soviet Committee for Struggle Against the Counter-Revolution] had to mobilise the worker-soldier masses. But the masses, insofar as they were organised, were organised by the Bolsheviks and followed them. At that time, theirs was the only organisation that was large, welded together by an elementary discipline, and linked with the democratic lowest levels of the capital. Without it the committee was impotent. Without the Bolsheviks it could only have passed the time with appeals and idle speeches by orators.

This popularity increasingly translated into votes to the soviets, facilitated by the practice of recall elections. By the end of September the Bolsheviks were winning majorities in both Petrograd and Moscow soviets. In the Second All-Russian Congress of Soviets (25 October, OS) they were the largest party, holding an outright majority by some counts. The Mensheviks and Right SRs subsequently walked out of this congress, allowing the creation of an all-Bolshevik government.

So the key difference between February and October is the Bolsheviks' ability to capitalise on the growing worker frustration with the Provisional Government and its policies. Not only had the Bolsheviks usurped the Mensheviks' role as foremost party of the working class but, following the April Theses, they had arrived at a novel Marxist justification for seizing power via the soviets. When they displaced the old Soviet leadership in October (at the Second Congress) this is exactly what they did.

As for the old leadership, they resisted until the end. Both Mensheviks and Socialist Revolutionaries split into those who accepted the legitimacy of Soviet power in October (the Left) and those who didn't (the Right). The major leadership figures tended to be part of the latter camp and they first pinned their hopes on the Constituent Assembly before generally passing into open counter-revolution or exile.

Reading

Most of this is covered in the general histories. I tend to recommend Rex Wade's Russian Revolution as the best introductory work around today.

On the soviets themselves, Oskar Anweiler's The Soviets is pretty old now, and somewhat outdated in places, but probably still the best study of the pre-revolution evolution of the soviets and the parties' attitudes towards them.

Specific quotes came from Michael Hickey's Competing Voices from the Russian Revolution and Nikolai Sukhanov's Personal Record. The latter is still essential reading for any student of 1917. The Robert Service reference is to The Bolshevik Party in Revolution 1917-23, his first and best work on the revolution.