To understand why Transylvanian Saxons preserved their own German identity we need to look back how exactly they got there and under what circumstances.

The first German colonists started arriving to Hungary at the invitation of Géza II around 1150 to strengthen the southeastern frontier, possibly due to fearing the Byzantine Emperor, Manuel I's expanionist policies. By 1186 they likely had sizeable colonies since the tax they payed in was substantial: „de alienis hospitibus de Ultrasylvas XV milia marcarum”. This trend continued with a steady stream of Germans hospes (term used in Hungarian records: latin for guest/newcomer) arriving to Transylvania to settle down. At the time Transylvania was sparsely populated (a charter says these Germans arrived to a land that was deserta), but due to its position (controlling the Carpathian passes) and wealth of resources (especially salt mining) populating it became a key goal for Hungarian kings (Székelys were settled down earlier in another Transylvanian region due to similar reasons).

This process got a huge kick when in 1211 Andrew II invited the Teutonic Order to provide military services (by now the main concern were Cuman raids from the danubian lowlands). Long story short the knights were kicked out in 1223, for one because they were getting bit too autonome but also because Hungarian nobles disliked the wealth and power they were amassing as a foreign order. However the German colonists were allowed to stay, and to ratify this the Andreanum was issued by the king, a charter that gave them special privileges. They got excemptions from various legal and financial obligations to the king and Hungarian nobles; they were treated as one legal unit, their collectively owned their land and it could not be returned to the king; they could trade and hold markets freely, they got staple rights and the privileges of nobles could not be exercised in their land. In general outsiders couldn't nose into their own business and they were allowed to live their ways. Their duties became fixated: 500 silver mark tax per year, and levie soldiers to the king if requested.

If you want to name one thing that made Transylvanian Saxons different, its the Andreanum. The combination of privileges and rules outlined within meant Saxons did not develop the kind of societal division common in feudal Europe: Their peasents were free and not tied to nobles, and their nobles were required to pay tax too. The Andreanum was confirmed by all kings of Hungary all the way to up Joseph II in 1785. Other rights I mentionned jumpstarted another thing: Urbanization. You see staple and market right was very important at time, I don't know if this applies to all of contemporary Europe but it was absolutely in Hungary and was a privilege of but a few towns. Coupled with their rights to elect their own judges Saxonland became one of the most urbanized regions of the Hungarian Kingdom. Towns like Hermannstadt and Kronstadt became among the largest and wealthiest of the entire kingdom too, to show no less importancet he German name for Transylvania, Siebenbürgen refers to seven Saxon fortified towns.

These strong rights and degree of societal/cultural seclusion made Saxonland its own regional unit and their people kept their German identity all the way to the modern era. Other ethnicites simply did not settle down and mix much with them (Saxonland maintained a German majority into the XX. century). However I wouldn't stress the ethnic factor much. In the middle ages social class and religion counted much much more - as such Romanians had a big barrier with their Orthodox religion, while Hungarians being largely peasents would simply not mix with the burgher Saxons. Even if someone really wanted to move to their land they'd face several hurdles: Due to their peculiar rights it was nigt impossible for an outsider to buy land in Saxonland - even if someone died heirless it simply went back to the collective ownership of whichever exact commune it belonged to. Social and spatial mobility was low, most peasents in the kingdom were tied to their noble (in early times there wasn't even any legal mechanism to leave). There was little incentive to do it at any rate: their peasent/shepherd skillset would not help them, they were unlikely to have the capital to buy a house. As for nobles of any kind, they had too had very little reasons to move around as they lived where their land was.

In conclusion, Transylvanian Saxons enjoyed a a regionally unique status which fostered a set of ways of living that promoted an identity, cohesion, was exclusive in nature and not invitive to outsiders.