Well, due to the Dark Forest Hypothesis, the trisolarians may fear that an entire planet being wrapped up for a long time may cause suspicion from the technologically hyper-advanced civilisations, causing the Earth and Trisol to be wiped out, like what happened in the third book.

The accident that happened at Trisol was already a risk too large for the Trisolarians to manage.

Besides, the second book shows that the sophon cannot propel itself or maintain its shape after two-dimensionalization. This makes it vulnerable to ground attacks. The following text is copied from a pirate version.

但他随后得知，智子在二维展开后没有任何空间机动和定位能力，只能以行星的引力为骨架保持形状，如果在太空中展开，将很快在太阳风等因素的作用下失去平面形状折叠起来，这就是二维展开后的智子只能在包裹三体行星的情况下才能保持形状进行电路蚀刻的原因。

The deepl translation is as follows. Sadly, I do not own an English copy of the book.

But he then learned that the sophon had no spatial maneuvering or positioning ability after two-dimensional unfolding, and could only hold its shape with the gravitational pull of the planets as its backbone, and if unfolded in space, would quickly lose its planar shape and fold up under the action of the solar wind and other factors, which was why the sophon could only hold its shape after two-dimensional unfolding to be circuit-etched if it wrapped around the three-body planets.

My old hypothesis:

Well, due to the Dark Forest Hypothesis, the trisolarians may fear that an entire planet being wrapped up for a long time may cause suspicion from the technologically hyper-advanced civilisations, causing the Earth and Trisol to be wiped out, like what happened in the third book.

The accident that happened at Trisol was already a risk too large for the Trisolarians to manage.

What I agree with now:

The second book shows that the sophon cannot propel itself or maintain its shape after two-dimensionalization. This makes it vulnerable to ground attacks. The following text is copied from a pirate version.

但他随后得知，智子在二维展开后没有任何空间机动和定位能力，只能以行星的引力为骨架保持形状，如果在太空中展开，将很快在太阳风等因素的作用下失去平面形状折叠起来，这就是二维展开后的智子只能在包裹三体行星的情况下才能保持形状进行电路蚀刻的原因。

The deepl translation is as follows. Sadly, I do not own an English copy of the book.

But he then learned that the sophon had no spatial maneuvering or positioning ability after two-dimensional unfolding, and could only hold its shape with the gravitational pull of the planets as its backbone, and if unfolded in space, would quickly lose its planar shape and fold up under the action of the solar wind and other factors, which was why the sophon could only hold its shape after two-dimensional unfolding to be circuit-etched if it wrapped around the three-body planets.

Actually, Luo Ji was concerned about Trisolarians messing with the broadcast via blocking the sun, so maybe temporarily blocking a planet or even a sun doesn't matter that much.

Well, due to the Dark Forest Hypothesis, the trisolarians may fear that an entire planet being wrapped up for a long time may cause suspicion from the technologically hyper-advanced civilisations, causing the Earth and Trisol to be wiped out, like what happened in the third book.

The accident that happened at Trisol was already a risk too large for the Trisolarians to manage.

Well, due to the Dark Forest Hypothesis, the trisolarians may fear that an entire planet being wrapped up for a long time may cause suspicion from the technologically hyper-advanced civilisations, causing the Earth and Trisol to be wiped out, like what happened in the third book.

The accident that happened at Trisol was already a risk too large for the Trisolarians to manage.

Besides, the second book shows that the sophon cannot propel itself or maintain its shape after two-dimensionalization. This makes it vulnerable to ground attacks. The following text is copied from a pirate version.

但他随后得知，智子在二维展开后没有任何空间机动和定位能力，只能以行星的引力为骨架保持形状，如果在太空中展开，将很快在太阳风等因素的作用下失去平面形状折叠起来，这就是二维展开后的智子只能在包裹三体行星的情况下才能保持形状进行电路蚀刻的原因。

The deepl translation is as follows. Sadly, I do not own an English copy of the book.

But he then learned that the sophon had no spatial maneuvering or positioning ability after two-dimensional unfolding, and could only hold its shape with the gravitational pull of the planets as its backbone, and if unfolded in space, would quickly lose its planar shape and fold up under the action of the solar wind and other factors, which was why the sophon could only hold its shape after two-dimensional unfolding to be circuit-etched if it wrapped around the three-body planets.