Short answer Because during Pratikraman the Kayotsarga (standing still, body-abandonment) is kept specifically for the praise (stuti) section of the Logassa Sutra. In many Śvetāmbara paddhatis you stand in Kayotsarga from the start of Logassa and relax the posture at the pad “chandesu nimmalayara” (wording may vary by pathshala/gacch). This maryādā was set so that:

• the devotee remains steady and humble while offering vandanā to the Tirthankars,
• laypeople do not strain the body beyond need; the rest of the recitation continues seated,
• a uniform cue exists in group Pratikraman.

Essence behind it

• Kayotsarga creates internal equanimity and detachment, aiding pratikrama (repentance) and nirjarā (shedding of karmas).
• Standing only for the initial stuti ensures discipline yet compassion toward the body, keeping focus on the soul rather than on physical discomfort.
• Elders, sick, or those unable may shorten or sit; the purpose is bhāva-shuddhi (purity of intent), not endurance.

Traditions

• Śvetāmbara: common maryādā to keep Kayotsarga till “chandesu nimmalayara,” then sit and complete the remaining sutra/avasyakas.
• Digambara: Kayotsarga is central too, but timings/verses differ; there isn’t this exact “chandesu…” stopping cue in their Pratikraman paddhati.

For simple overviews, see:

• Pratikraman (purpose and steps)
• Logassa (what it is and its role)