Fundamentally, because Byzantium’s traditions of succession were completely different in both theory and practice when compared to the western medieval states.

The sharpest and most recent scholarship on this matter sheds light on that difference with great clarity, but before I get into that I want to make two points of distinction. Firstly, many of those attempted usurpations — perhaps even most — had more of the tenor of a political campaign than an out-and-out civil war. While this type of Byzantine usurpation could involve some serious fighting, some serious recent scholarship has been used to convincingly argue that Byzantium retained many characteristics of a semi-republican system. Essentially, a claimant had to acquire and cement the support of several independent elements within Byzantine society to be seen as legitimate — the army, the rural landowning nobles, the agrarian smallholders, the clergy and the citizens of Constantinople were all different parts of one single body politic. The Principate and Dominate structures of the Classical and late Antique Roman era had never really been a dynastic hereditary monarchy de jure, and elements of that ancient tradition remained strong in the New Rome.

Many of the revolts in the middle Byzantine period have almost the shape of an election — when Bardanes Turcus mounted his own unsuccessful attempt, he and his retainers/a segment of loyal soldiers approached the capital. They made no real attempts to take the city itself, but instead made a public demonstration to see if the Byzantine citizenry and nobility were tired enough of the current Emperor to open the gates and let Turcus in. Regrettably for him, they were not, and Turcus’ revolt fizzled out as his men abandoned him.

Sometimes a battle would be required to quash men like Turcus but often political maneuvers were more than enough. Constantinople’s impregnability meant that even relatively unpopular Emperors could weather any usurpation even if much of the empire despised them. Such examples are rarer, and take on the aspect of an actual civil war; Scleros and Phokas’ wars against Basil the Bulgar Slayer come to mind, as does the massive revolt of Thomas the Slav against Michael of Amorion. The standout example of an actual civil war in the century you mention is Isaac Komnenos’ great uprising — and arguably that war was as much about an actual ideological conflict between the rotating class of incompetent court emperors and the military nobles who saw the empire’s borders breaking down day by day. Wars like this are notable by their comparative rarity.

Secondly, you mention the 11th century, and that is indeed a well chosen era for this question — primarily because with the extinction of the glorious Macedonian dynasty no candidates had any inborn legitimacy. When added to the chaos after Manzikert, the situation grew yet more dire. Under normal conditions, the Byzantine system could just about ensure competent leadership; even well established dynasties could falter with an unworthy heir, because the nature of the ‘republic’ meant that support was easily shifted to a newer and more ambitious/skilled candidate. However, when foreign forces were added to the mix the strength of the Byzantine system’s succession could become a weakness.

It was only under superlatively strong emperors like Alexios Komnenos and his heirs, or the various Macedonian rulers (though even under this dynasty weak rulers were often subjugated by competent co-emperors like Romanos Lekapenos) that the empire took on the trappings of a dynastic monarchy. The western Crusaders and visitors were baffled by this distinction, and indeed it was their horror at the entirely routine deposition of Alexios Angelos that provided the impetus for the disastrous 4th crusade.

This system could be deeply unstable, but in better times it did mean that on average the sitting Emperor was often well-equipped to rule and, by natural selection, the most ruthless and determined man or woman of the day.

Sources: Two fantastic works by Anthony Kaldellis, the Byzantine Republic and also Streams of Gold, Rivers of Blood.