It is because the bible says so.
It is because I believe in what the bible says.
Seredipitously, your reply highlights your problem. Let us assume you and I are 1st Century Christians who are discussing the idea of Mary’s sinlessness. What scripture would we rely on? All we have as sacred scripture, the TANAKH or OT. There was no New Testament.
But let’s fast forward and assume we are 2nd/3rd Century Christians. In discussion you might quite legitimately support your case with verses from, say,the Gospel of Peter or of Thomas. I might counter with verses from the Gospels of Philip or of Mary Magdalene. These gospels existed and were in use by early Christians. Where is Sola Scriptura in all this???
Of course the NT we have today and which virtually all Protestant denominations accept as sacred scripture wasn’t defined as Canon until the 5th Century following a number of Councils - of the Catholic Church!! And even then the idea of Sola Scriptura wasn’t even on the radar - if you’ll excuse the anachronism.
If you accept things ‘because the Bible says so’, then you have a major problem:
Scripture tells us Christ left a Church with ‘divine authority to govern in his name’
(Mt 16:13-20, 18:18; Lk 10:16)
And, of course, he promised his Church would last until the end of time (Mt 16:18, 28:19-20; Jn 14:16-18)
In addition you have to explain away 2 Thess 2:15, 3:6 where the Bible tells us (and you!) that Sacred Tradition is to be followed alongside Sacred Scripture "…hold fast the
tradtion which you have bee taught
whether by word or our epistle.
So what is this tradition, passed on by word? Clearly it isn’t written in the Bible.
And, of course, you can quote
nothing from the Bible which talks of the doctrine of sola scriptura!!! By your own logic if the Bible doesn’t tell us then it can’t be true. In fact, the Bible tells us we need more than just the Bible alone! The Bible confirms that not everything Jesus said and did is recorded in scripture (Jn 21:25) and that we must hold fast to oral tradition, the
preached Word of God (1 Cor 11:2; 1 Pet 1:25)
In 2 Pet 3:15-16 we are warned that Sacred Scripture can be very difficult to interpret, which strongly suggests the need for an authoritative interpreter (not just Richard Kastner who interprets on his own authority!?)
Finally, 1 Tim 3:15 reassures us that the Church is the ‘pillar and bulwark of truth’.
Now bearing in mind all this is in the Bible how do sola scriptura afficionados explain it all away? I guess one or two passages could be given bizarre interpretations but, taken as a whole, it is manifestly clear that sola scriptura is an unsupportable doctrine - not least because sola scriptura itself is nowhere mentioned in the Bible.