Why is the Buddha described as trackless?

This is actually a mistranslation of the verse; Buddhist Legends by Burlingame is a bit better:

The Buddha, unlimited in power, the trackless.
By what track can you lead him?

In order to understand this verse, it might help to see the context; the Buddha is using poetry and imagery. Here's the Pali leading up to the verses in the commentary (it's about Mara's daughters):

puna māradhītaro “uccāvacā kho purisānaṃ adhippāyā, kesañci kumārikāsu pemaṃ hoti, kesañci paṭhamavaye ṭhitāsu, kesañci majjhimavaye ṭhitāsu, kesañci pacchimavaye ṭhitāsu, nānappakārehi taṃ palobhessāmā”ti ekekā kumārikavaṇṇādivasena sataṃ sataṃ attabhāve abhinimminitvā kumāriyo, avijātā, sakiṃ vijātā, duvijātā, majjhimitthiyo, mahallakitthiyo ca hutvā chakkhattuṃ bhagavantaṃ upasaṅkamitvā “pāde te, samaṇa, paricāremā”ti āhaṃsu.

tampi bhagavā na manasākāsi yathā taṃ anuttare upadhisaṅkhaye vimuttoti. atha satthā ettakenapi tā anugacchantiyo “apetha, kiṃ disvā evaṃ vāyamatha, evarūpaṃ nāma vītarāgānaṃ purato kātuṃ na vaṭṭati. tathāgatassa pana rāgādayo pahīnā. kena taṃ kāraṇena attano vasaṃ nessathā”ti vatvā imā gāthā abhāsi —

Burlingame translates this as follows:

Said the daughters of Māra again, “Many and various are the tastes of men. Some like maidens, others like women in the prime of life, others like women who have reached middle life, while still others like women who have passed middle life. We will tempt him in various forms.” So one after another, they assumed the forms of women of various ages, creating by supernatural power each a hundred female forms. And in the guise of maidens, women who had not yet given birth to a child, women who had given birth to one child, women who had given birth to two children, women who had reached middle life, and women who had reached old age, they approached the Exalted One six times and said to him, “Monk, we would be your humble slaves.”

But neither did the Exalted One pay any attention to that, but remained free, even as though the elements of being had been utterly destroyed. [197] But when, after that, they did not retire, the Teacher said to them, “Depart; what do you see, that you strive thus? Such actions as these should be performed before those who have not rid themselves of the lusts and other evil passions. The Tathāgata, however, has rid himself of the lusts and other evil passions. Why will ye try to bring me into your control?” So saying, he pronounced the following Stanzas,

The Buddha's answer in the verses includes the following statement:

taṃ buddhamanantagocaraṃ,
apadaṃ kena padena nessatha.

The actual literal translation is:

That Buddha whose pasture is limitless;
pathless, by which path will you lead?

The line is in regards to the imagery of a creature not limited by fences, thus with unlimited gocara (lit. pasture). Such a creature cannot be led anywhere, since it is not possible to contain them.

So, the mistranslations are as follows:

gocara should be "pasture", not "range" (it loses the imagery, if nothing else) and certainly not "power"

nessatha should be "lead", not "trace"

So, what is the actual meaning?

The commentary says:

kena padenāti yassa hi rāgapadādīsu ekapadampi atthi, taṃ tumhe tena padena nessatha. buddhassa pana ekapadampi natthi, taṃ apadaṃ buddhaṃ tumhe kena padena nessatha.

By what path - whereas those for whom there is even one path out of [all path] starting with the path of lust, them you may lead by that path. But for the Buddha there is not even one path; that pathless Buddha - by what path will you lead?

The meaning is clear; while it is possible to lead ordinary worldlings on by means of the carrot or the stick of defilements, there is no leading on of enlightened beings who have no such defilements.

A creature with a defined pasture can be led because of fences and ropes. A creature with a limitless pasture (like the Buddha) cannot be so led. The idea is then of not being drawn by defilements and thus with no definable path that mara can limit or goad one down.

Ultimately, it's just poetry (ancient poetry at that), and should not be taken literally. A proper understanding of the use of the word pada might require a knowledge of Maghadese colloquialisms 2500 years ago.