From and article by Rav Meir Shpiegelman in Yeshivat Har Etzion's Daf Kesher #114.

We are familiar with the gemara on Shabbat 21b which lays out the manner in which the mitzva of lighting Chanukah candles is to be performed. According to Beit Hillel, we subscribe to the principle that "we ascend in holiness and do not descend," and thus we light one candle on the first night, two on the second, and so on until eight. What the gemara does not explain is why the baseline is one candle and why we add one every night (why not more?). Why should we not light eight candles every night? Even if we say that we are specifically trying to demonstrate this principle of ascending in holiness, we still need to explain why this is so.

When the Chashmonaim entered the Temple after defeated the Syrian-Greeks, they were faced with two possibilities (we are, of course, assuming that they did not know that a miracle would occur, and that they knew that it would be eight days before fresh oil arrived): they could light all of the candles on day one and finish the oil, or they could divide the oil and light one candle each night until more oil came. The second option was contingent on two things: whether the mitzva of lighting the menorah in the Temple could be done with only one light lit, and even if it could, since there was certainly a higher fulfillment to light all of the candles in the menorah, should they light all of them on day one, even though they would not be able to light for the next seven nights?

Our answer begins in the psukim in the Torah that speak about the lights in the Temple and Tabernacle. While there are many, we will focus on four major ones:

1. Shemot 25:37 presents us with the instruction to build a menorah, stating that its purpose will be to provide light.
2. Shemot 27:20-21 presents the commandment to make pure olive oil, for the purpose of having a permanent light (ner tamid) lit within the Tabernacle.
3. Vayikra 24:1-4 reiterates the commandment to make pure olive oil, but now adds the fact that it should be used specifically in the menorah (which is not specified in Shemot 27).
4. Bamidbar 8:1-4 presents the commandment to Aharon to light the menorah every day.

Already at first glance, the differences among these verses are apparent. The second verse makes no reference to the menorah at all, but merely states that a ner tamid must be somewhere in the Tabernacle. This omission is highlighted by the fact that the third verse is nearly identical to the second, yet it includes the menorah. This verse in Vayikra can perhaps be seen as adding on to the one in Shemot, and thus tells us that the ner tamid in the Tabernacle was to be lit on the menorah. The first verse focuses not on the candles, but rather on the menorah itself, and presents the lighting of the candles as an act which flows from the physical menorah, and not from an independent commandment.

 Using the verses, we can explain several laws that are connected to the menorah, of which we will elucidate three here:

 If we now look more closely at the commandments in the Torah, we will see that each one adds another detail to the law. First we learn that the menorah needs to have candles. Then we learn that in the Temple a candle has to be lit all night. That candle has to be situated on the menorah (and might even have to burn during the day). Finally, the lighting of the candles has to be done by Aharon and his descendants.

The Chashmonaim who entered the Temple found enough oil to last one day (oil for each of the seven candles, and an eighth portion that was needed as a backup for the "western candle" lest it go out during the day [Rashba]). The menorah, like any other vessel in the Temple, only attained its full level of holiness and status as a Temple vessel after having gone through the process of "chinuch" – dedication and consecration, a process that was accomplished through the lighting of all seven candles. As such, the Chashmonaim decided to not consecrate the menorah and to only light one candle on the first night. Had they lit all seven right away, they would have been refraining from doing a positive commandment when they would be unable to light on the second night. However, if the menorah remained unconsecrated, that commandment would not yet exist, as the commandment to light all of the candles, as we have seen, is derived from the menorah itself. Thus, if there was a lacking in the menorah, there could not be a commandment to light all seven candles every day. Rather, they lit only the ner tamid, which was a commandment which was independent of the status of the menorah and thus had to be lit regardless. The first miracle thus came when that candle stayed lit all day, as was the "normal miracle" in the Temple. This miracle showed that Hashem was accepting the actions of the Chashmonaim and that things were going to return to their normal way in the Temple.

The fact that we add a candle very night is a reflection of a second miracle that occurred. As it is not as important as the initial one, it is not incumbent upon everyone to do. At the end of the first day in the Temple, the oil in the one candle had not gone out. Thus, on the second day the Chashmonaim were able to place oil into the second candle alongside the still-burning first candle. This continued for seven days, and only on the seventh night was the menorah fully consecrated. The eighth night was thus the first night that all of the candles were lit together as they would be on any other day (similar to the eighth day by the consecration of the Tabernacle – see Vayikra 9). The consecration occurred on the seventh night and on the eighth night the newly-consecrated menorah was first put to use. All of the candles were lit that night by way of a miracle (that the oil continued to burn), and thus while the menorah looked normal to the outside observer, its light was entirely the result of a miracle.