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After the Enterprise engages with the Borg cube, it witnesses a Borg sphere heading towards Earth and then go back in time.

Picard knows all too well the Borg are not allies and any Borg entering federation space must be investigated and handled accordingly.

Why didn't he deploy a tractor beam to stop the sphere from moving?

His ship and others just came off a fight with the Borg cube, they may not have sufficient weapons to attack the sphere, but they should've have held it in place and call for backup.

Tractor beams are easily shaken off and cannot work if the target has their shields up. While the Cube was destroyed, the Sphere appeared undamaged when it came out of the Cube and headed towards Earth. While it wasn't specifically stated in the movie or in the novelization, it is highly likely that it had it's shields up, so a tractor beam would have been useless.

It is stated, once the Enterprise emerges from the temporal vortex, that, among other systems, their shields are down due to the shaky trip through the vortex. The Sphere probably had the same problem since one volley of four quantum torpedoes could destroy it.

Tethering themselves to the cube would pose a higher risk of being boarded, and assimilated (resistance is futile, we are borg, you will join the collective, your mind will become our mind, we are borg... Sort of thing) Also, having the tractor beam touch the cube poses the risk of their technology adapting, with their hyper analysis granted to them via a collective mind. It would only be a matter of minutes before they would break free or even find a way to reverse the tractor beam and utilize it as a weapon.

Riffing off Loki's comment, there is also the consideration that there may not have been time to both think of and implement that idea. Quoting the script, which doesn't have timestamps but from what I remember this all happens pretty quickly:

(the Enterprise has just arrived at Earth)

PICARD: This is Captain Picard of the Enterprise. I am taking command
of the fleet. Target all of your weapons onto the following
coordinates. ...Fire on my command.

DATA: Sir, the coordinates you have indicated do not appear to be a
vital system.

PICARD: Trust me Data.

RIKER: The fleet's responded, sir. They're standing by.

PICARD: Fire.

(the fleet attacks and a circular aperture opens in the Borg ship and
a Borg sphere launches before the ship explodes)

Given that 'the fleet' is firing, it's probably OK to say the Enterprise is at a reasonable distance from the sphere:

PICARD: Mister Hawk. Pursuit course. ...Engage.

Confirming our guess. You don't have to pursue something if you're already close enough to... do whatever you want to do. Tractor beams have a relatively short range, from what I remember.

[snip a few lines of continuous dialogue, no great time passes]

DATA: Sensors show chronometric particles emanating from the sphere.

PICARD: They're creating a temporal vortex.

RIKER: Time travel!

(the Enterprise suffers a shockwave)

PICARD: Data, report!

DATA: We appear to be caught in a temporal wake.

WORF: Captain, ...Earth.

[at this point Earth is put on screen, everyone looks up and gasps, because it looks like it's completely metalled over]

DATA: The atmosphere contains high concentrations of methane, carbon
monoxide and fluorine.

PICARD: Life signs?

DATA: Population ...approximately nine billion. ...All Borg!

So less than a minute after we started pursuing them, already they have changed history. What use a tractor beam now?