There are two fundamental disputes in hil tefilah that seem to go hand in hand:
1) There is a machlokes Rambam and Ramban whether tefilah is a mitzvah d’oraysa once a day every day (Rambam) or whether tefilah is a takakah derabbanan to be fulfilled multiple times a day (Ramban).
2) The gemara has a machlokes whether tefilas arvis is reshus or not, i.e. whether the obligation to daven ma’ariv carries the same weight as the obligation to daven the other tefilos.
Putting two and two together: According to the Rambam, who holds that tefilah is d’oraysa, the first opportunity in the day to fulfill that chiyuv is ma’ariv. It doesn’t seem to make sense to say that ma’ariv, which is potentially a kiyum d’oraysa, would be only a reshus, but other tefilos would be chovah! The view that holds ma’ariv is reshus seems to only work if you hold tefilah is derabbanan; the view that holds ma’ariv is chovah fits nicely if you holdstefilah is d’oraysa.
In one of his recent Motzei Shabbos shiurim, R’ Yitzchak Yosef pointed out that while the lomdus sounds nice in theory, it's dead wrong. As noted, the Rambam paskens that tefilah is d’oraysa (hil tefilah 1:1):
מצות עשה להתפלל בכל יום
Yet the Rambam also holds that tefilas arvis is reshus (hil tefilah ch3):
תפילת הערב--אף על פי שאינה חובה--המתפלל אותה, יש לו להתפלל מתחילת הלילה עד
שיעלה עמוד השחר.
Somehow you need to explain how to fit the two together. That’s a homework question.
Why is it that tefilas arvis is reshus as opposed to the other tefilos?
According to one view in Chazal, our tefilos correspond to the avodah done in the Mikdash: shacharis corresponds to the korban tamid done in the morning; mincha corresponds to the tamid in the afternoon; ma’ariv corresponds to the fats and flesh left burning on the altar during the night. I think we can agree that it makes sense for there to be a distinction between teflios that correspond to the korbanos tamid, where there was a mitzvah to offer a korban and sprinkle the blood on the mizbeiach, and ma’ariv, where there was nothing to do other than leave the remaining fats and meats on the altar to burn.
But Chazal also tell us that tefilos were instituted by the Avos. Avraham instituted shacharis; Yitzchak instituted mincha; Chazal darshen from the pasuk “Vayifga ba’makom” in this week’s parsha that Ya’akov instituted tefilas arvis. According to this view, why is ma’ariv different than the other tefilos?
The Shem M'Shmuel answers by quoting the Midrash on "Tzamah lecha nafshi, kameh lecha besari” that explains that just as Ya’akov’s nefesh thirsted for G-d, so too did the 248 limbs of his physical body. Ya’akov succeeded in obliterating the divide between guf and nefesh. There was no longer within him a dichotomy, a struggle, between physical self and spiritual self – it was all one unit dedicated to avodas Hashem.
This level of avodah is one that only the elite can reach; it cannot be a chovah incumbent upon all.
Rather than see these as two different approaches -- tefilos as representative of avodah in the Mikdash vs. tefilah as an institution of the Avos -- the Shem m’Shmuel suggests that there is an underlying harmony between the views. Avraham and Yitzchak succeeded in worshipping G-d with all their soul; Ya’akov succeeded in worshipping with his entire being. The most essential avodah of all korbanos, including the twice daily tamid, was the zerikas ha’dam, splashing the blood on the mizbeiach. “Ki ha’dam hu hanefesh,” blood represents the soul, spiritual self-sacrifice. At night, however, it was the flesh and fats, even the physical elements of the animal, that were able to be consecrated and burned on the mizbeiach. This is tefilas ma'ariv, the avodah of Ya'akov, who was able to unify body with soul in his worship.